Watershed

In UK broadcast law, watershed is the time after which "adult" content, i.e. programs with a lot of intense violence, coarse language, 'naughty' bits, etc. can be shown. The same concept is named safe harbor in US FCC regulations. (It's like they're trying to make you need to get up and go to the bathroom – or perhaps the loo – with all this hydraulic terminology.)

Usually 9 PM in the UK, the US and Canada. Somewhere between 8.30 and 9.30 pm in Australia. In Japan, which has more liberal views on acceptable content to begin with, the "Otaku Hour" starts at "25 o'clock" (1 AM). In Germany that would be 10 p.m. for movies free for age 16+ and midnight for 18+. The UK has no such watershed for radio, so a long list of words can't be said under any circumstances.

One interesting thing a character cannot do before UK watershed is die with their eyes open. (Although, for some reason, being already dead with your eyes open is fine.)

In the US, the FCC rules remove some of the content restrictions on broadcast TV and broadcast radio between 10:00 P.M. and 6:00 A.M. This allows "indecent" (R, and even NC-17 rated) programming to be shown uncensored on regular networks. They can play "offensive" material, such as George Carlin's "Seven Words You Can't Say on Television," but "obscene" material (which has a specific legal definition) is still prohibited. Examples of "obscene" material would be child pornography and certain forms of hard-core adult pornography. There are no restrictions on cable or satellite TV stations or satellite radio.

In practice? Thanks to fickle advertisers who don't want to be associated with risqué content, this rule is seldom exploited, at least in the US. A notable exception is PBS, which, thanks to a combination of its funding system and its audience, tends to just let things fly on their post-Safe Harbor programming, particularly with shows imported from Britain.

Watershed is named for the earthen barriers at the edges of a farm field, which prevent irrigation water from crossing to a neighboring field. This allows two adjacent fields to be on very different watering schedules so that different crops can be planted there. When driving past a farm on the highway, you'll see row after row of (say) asparagus, then a watershed will pass and suddenly you're seeing row after row of rutabagas.

United Kingdom

 * For a while in the UK, the watershed seemed to be 9:30. There were episodes of Hell's Kitchen where Chef Ramsay would be bleeped for the first half hour, but not the second. Quite amusing.
 * The watershed is not meant to be an absolute dividing line where anyone with any sensitivities should instantly stop watching, because it's recognised that people sometimes leave the TV on just to see what's next. Channels have been reprimanded when they've shown something at 9:00 which immediately starts with a Cluster F-Bomb or a graphic sex or Gorn scene.
 * Conversely, films rated 15 have been known to be screened with an 8:30 start on the rationale that most of the film will indeed be after the watershed. They'd be unlikely to get away with bending the rules like this for a film rated 18, though. Sky Movies has an 8 pm watershed for 15 films and a 10 pm watershed for 18 films.
 * The rules are slightly more relaxed for pay TV, although not much. A free-to-air channel would struggle to get away with the same thing.
 * Doctor Who (and Torchwood, by extension) seems to be pushing the envelope as far as it can to see what it can get away with pre-Watershed. The parent programme is aired in one of the most successful television spots in the country (Saturday evening) and can get away with quite a lot including male-on-male kissing, dying with eyes open, several bondage scenes and Davros' shirtless scene. The second series of Torchwood had watered-down airings of episodes at 7pm on Wednesday, but was still quite risqué.
 * Doctor Who has always done this, with its infamously high body count. The mid seventies in particular was especially graphic, often showing impalements, decapitations and blood squibs with brutally graphic detail.
 * On the subject of Doctor Who, Tennant's portrayal of Hamlet was aired in the early evening during Christmas 2009. Their interpretation of the famous Country Matters line was so heavy handed you wonder how they got away with it.
 * If it's Shakespeare, it's educational!
 * UK police drama The Bill is an interesting case study. It started out in 1984 as a series shown in the late evening, after the watershed, meaning it had liberal cases of sex, violence and nudity (it was in this period that it was at its most gritty and realistic). In 1988 the production team decided to move the programme to be broadcast before the watershed, which meant such things were quickly toned down and, eventually, removed altogether. It ran in this pre-watershed timeslot for most of its life, however in 2009 the decision was made to move it back to being broadcast after the watershed, at 9pm.
 * The watershed was outright invoked on Top Gear in the "Top Ground Gear Force" special, since the special aired beyond the watershed when the show's normal timeslot is before it. A very annoyed James May stopped to verify the special was airing beyond the watershed before dropping a Precision F-Strike. It was still cut off by a hard cut to Richard Hammond, though.
 * One of the most notable breaches of the watershed in Britain was the infamous Bill Grundy interview with the Sex Pistols in 1976. Grundy provoked them into saying all sorts of swear words - during prime time viewing hours.

United States

 * US terrestrial TV tends to go out live in New York, with a two-hour delay in Denver and a three-hour delay on the West Coast.
 * Terrestrially, Chicago is an odd backwater which lags Thunder Bay, Ontario by a full hour in time zone... yet they're given the same live feed as the US East Coast. That has some not-so-subtle effects: a network prime-time lineup which follows the 6/6:30PM news doesn't start until "eight, seven Central" with the New York stations stuffing syndicated fare into the resulting hour "local availability" hole. It also shifts the 9PM watershed to be "10 Eastern, 9 Central", possibly pushing the entire watershed to 10PM in the west as they're running a fixed-delay version of whatever already went out at 10 ET in NYS.
 * The fact that North America (the US and Canada especially) spans over a very large number of time zones makes the issue of a watershed time problematic. In Hawaii, Cartoon Network's Adult Swim starts at 7:00 PM.
 * EST has Cartoon Network West, which airs Adult Swim until 9 AM. Family Guy is on at 6 and 6:30 AM.
 * Los Angeles' CW affiliate is available on cable and satellite TV on the west coast going in as far as Arizona, which doesn't observe Daylight Saving Time.
 * Because of this, "post-Watershed" in America doesn't mean a whole heck of a lot, especially on Network Television. NYPD Blue gave us the occasional flash of male backside, but for the most part, post-Watershed shows might get away with the occasional utterance of "Bitch" and that's about it. On the other hand, cable is more relaxed than network, pay channels like HBO are more relaxed than cable, and the levels of what they get away with varies accordingly.
 * Comedy Central shows uncensored Stand-Up and movies in their Secret Stash. Comedy Central also shows commercials for "Girls Gone Wild", phone sex networks, and pay per view porn at that time. Indeed, they broadcast one or more of these ads every commercial break after 10:00. It can get a little annoying. This has now gone through the looking glass, to where the newer stand-up acts aired during these time slots will actively reference that the viewer will likely see at least one Girls Gone Wild commercial during the breaks.
 * Although they claim that their movies are uncensored, some still are. Nudity is normally blurred out. They lie.
 * Most of the comedy specials debuted first on pay TV stations like HBO which are not covered by F.C.C. censorship rules.
 * PBS's adaptation of the play The Bridge Game, in which Mary Tyler Moore and Dick Van Dyke drop F-Bombs on each other for five minutes.
 * When Conan O'Brien moved from 12:35/11:35 to 11:35/10:35, a big deal was made about how some of his show's racier skits could not go with him. Sort of. The biggest offender everyone could think of was the Masturbating Bear, which actually showed up and did his thing on the show's prime-time Milestone Celebration in 2003. He also showed up on The Tonight Show when Conan got sick of NBC's extreme Executive Meddling. Conan's going to cable, so he'll be much freer content-wise.
 * Pretty much the point of Adult Swim, after a fashion: As noted above, the FCC doesn't regulate what cable channels show. Even so, advertisers and network executives expect that kids won't be watching TV (not even Cartoon Network) during Safe Harbor, so they're willing to let Adult Swim do what it does during that timeframe. Hell, nowadays, it sometimes airs from 10:00 PM to 6:00 AM exactly.
 * A kind of predecessor was Toonami's "Midnight Run" showing of Gundam Wing, without most of the censorship shown in the afternoon broadcast earlier in the day.
 * A careful analysis of Spike TV's advertising during the Safe Harbor reveals that the network apparently assumes its entire viewership to be male, between 18 and 30, undereducated, unemployed or underemployed, and pathetically horny.
 * To be fair that is why the channel was created.
 * NYPD Blue was one of the few network TV shows that exploited this.
 * Lampshaded in The Late Late Show, especially when Craig Ferguson does a lot of (censored) swearing.
 * According to the censors, Geoff Peterson can get away with a hand gesture resembling masturbation because he doesn't have genitalia.

Canada

 * Unlike the US (where programmes appear an hour earlier in the Central time zone) Canada is more likely to encounter weird timezone issues going East. The world will end at midnight, "half an hour later in Newfoundland" « et une heure plus tard dans les Maritimes » is an old running joke around CBC/Radio-Canada. This is less likely to impact watershed - as it merely pushes the programming further into the night on the Atlantic coast.
 * When Erin Brockovich was on CTV, because of the timeslot it was in and the length of the movie, the first half of the movie was censored and the second half was not. It was quite the surprise to be greeted after the commercial break with a triumphant "fuck you!"
 * Subverted by the CBC, which doesn't really seem to care about the watershed; they've shown language to make a sailor blanch on the six o'clock news. There are certain things only the MotherCorp can get away with.
 * Although the CRTC has hit them at least once.
 * The predecessor to LGBT community channel "Out TV" was "Pridevision" – an encrypted pay-TV channel which showed gay community programming by day and abruptly flipped to outright hardcore pornography at midnight with the bizarre disclaimer "This program deals with mature subject matter and contains scenes of explicit sexual activity, which may include coarse language. Viewer discretion is advised."
 * Coarse language? What the f*ck?
 * At one point (after time-shifted east/west feeds were scrapped, but before the porn was split to a second channel "Pridevision Hard" that both Bell and Shaw refused to carry) porn was going out at midnight Eastern, 9PM Pacific. As an encrypted channel, Pridevision was not subject to watershed.
 * City TV (Toronto) and other local TV stations in Canada used to schedule soft porn late at night. After Bell acquired CHUM Radio, CITY-TV was sold to Rogers. Now they don't air any porn at all.
 * Videodrome (1983) and "CIVIC-TV 83" is a fairly blatant cheap shot at the early CITY-TV 79 in Toronto.
 * At one point, is was possible to get away with slightly more in French than in English on Canadian TV. This was for historical reasons – the church once held a stranglehold on everyday life in rural Québec, only to meet with fierce backlash during the Quiet Revolution (« révolution tranquille ») in The Sixties. That left Montréal more liberated than Toronto on many questions, ranging from the criminalisation of abortions to the minimum age to buy alcohol to the censorship of film and prerecorded video, well into The Eighties.

Japan

 * Japan's Otaku Hour has brought several anime series based on "ero-ren'ai games" - incorrectly known as "hentai games" in the US. These are usually cleaned up for TV and shown during the Otaku Hour.
 * There are also live action dramas that play during Otaku Hour, usually with more extreme content than usual. Two examples spring to mind, both of which played on the same time slot on the same network—a live action version of the adult manga / anime Hen (known as "Strange Love" in the US), which had a scene of the two Schoolgirl Lesbians skinny dipping and making out in the school pool, and Invisible Girl Ea, revolving around the titular young woman Ea, who spends the entire 6 episode series very obviously nude with the excuse that clothing causes her invisibility powers to freak out.

Sweden

 * The Swedish version of MTV used to show rather explicit music videos, such as Everybody Knows That You're Insane by Queens of the Stone Age, which featured a naked guy running around in his apartment doing... unorthodox stuff.