Safe Word

My safeword is 'OW!'

—T-shirt slogan

Sometimes a word that really means "No" is needed, but the actual word "no" isn't available for one reason or another. A "safe word" can be aimed at the person who interacts with you, to underscore that you really need to hit the emergency brake. Or it could be meant for a third party that might come to your aid. Or both. This is especially true in BDSM, where a sub's sobs and pleas for mercy are often meant to convey that they do not want the dom to stop.

In the American BDSM subculture, a particular set of three safe words are widespread: Red for full stop emergency shutdown, yellow for "This is too hard, I need us to slow down," and green for "Don't mind my screams, you can push harder." Other countries' BDSM scenes can be considerably more lax, though, with some even foregoing safewords altogether and placing the responsibility on the dom to ensure that things don't get out of hand. Both approaches have their advantages: safe words allow more control over the acts and less actual harm inflicted but at the downside of having to go out of character every once in a while; conversely, dom control offers much more intense play at the cost of potentially overdoing it (and risking running afoul of sexual abuse laws).

Of course, any word can be used as a safe word. And the concept is not limited to sexual situations either. Commonly used by undercover operatives of all kinds while wearing a wire or being in a bugged room.

When Played for Drama, (sub)cultural flavor or similar, the safe word is likely to be either "red" or some other simple word that isn't easily used by mistake. When Played for Laughs, it is instead likely to be:

Something unlikely to be said in casual conversation.
Something hilariously unsexy, such as the name of an Ugly Politician Local to Area Where Writer Lives.
Something which can be confused easily with something else, leading to hilarious consequences when the safe word is invoked by mistake.
Something which is hard to remember or hard to pronounce, giving the character a really hard time.

Compare Safe, Sane, and Consensual. Contrast Trust Password.

No real life examples, please; for sexual or sexualized examples; this is All The Tropes, not Tropes After Dark. Non-sexual examples are okay.

Examples of Safe Word include:

Anime and Manga

Gundam Seed Destiny has a version of this that is both extreme and stupid. The the Earth Alliance's enhanced soldiers each possess a "block word" that's supposed to render them docile. What it actually does, however, is drive them completely nuts. To make matters worse, the two block words we hear are common words that one is very likely to hear on a daily basis: "die" for Stella and "mother" for Auel. In fact, the one time we see Auel subjected to his block word he blurts out Stella's during his mad ranting, making the situation that much worse.

Comic Books

Indirect in City of Dreams: Those who don't want to play anymore simply wake up.

Fan Works

A Buffy the Vampire Slayer fanfic^[context?] has the safe word "Hellmouth", chosen for its unsexyness.
A Harry Potter Song Fic for Rihanna's "S&M", where the safe word, and title, was Skittles.^[context?]
Another Harry Potter fanfic, called Roll Out The Red Carpet, plays with this trope; When Ron and Hermione role-play, they use the phrase "Jam Jar" as a sign to stop (as they acknowledge that, when playing particular roles, "stop" and "no" might just be them acting). They almost get into trouble when, upon almost getting caught in Hermione's office, Ron doesn't stop what he's doing because she didn't say jam jar and he thought she liked it, and it leads to a Crowning Moment of Heartwarming later on when Ron uses the safe word to make sure that Hermione's okay with what they're doing, and to check that he's not hurting her too much.

Film

Eurotrip features a combination of types 2 and 3 when Cooper unknowingly goes into a BDSM club, Vandersexxx. The safe word in question? "Flüggåɘnk∂€čœßøl∫ên." Yes, that's the Euro symbol and an integration sign in there.
Four Christmases: The main characters are visiting the parents. They decide to use the word "mistletoe" for when they cannot take it any longer and want an excuse to leave.
The Goods: Live Hard, Sell Hard has one character, after being bored with "making love," tells the girl he is with to do whatever she wants. His safe word is "Blueberry Pancakes."
Invoked in Serenity: the phrase that puts River to sleep is referred to as one of these. (A sleeping River is safer - to everyone else - than an awake-and-violent River is.)
Hot Rod: Rod chooses the word "whiskey" for his safe word when practicing for the big stunt at the end of the film. Unfortunately, not only can he not pronounce the word correctly, but his crew is unable to help him anyway.

Literature

In Catt Ford's A Strong Hand, the main couple use the safe words "London" (for stop) and "yellow" (for slow down).
Appears in The Last Hot Time by John M. Ford, as the hero learns about BDSM.
In Kushiel's Legacy, a safe word is known as a signale. To persist with BDSM play after a partner has given the signale is considered heresy according to their religion of "Love as Thou Wilt."
Played with in Callahan's Lady by Spider Robinson. In the Bower at Lady Sally's (a pretty much anything goes orgy room), there's only one safeword: "No".

Live-Action TV

The Office: Michael becomes Jan's partner for a period in the third season, but is unnerved by her many fetishes and fondness for roleplay. But he's too much of a wimp to leave her, even when she "forgets" their safe word. Luckily he manages to by the end of the episode.
Tracy on 30 Rock: "Boundaries are made to be tested. That's why my wife and I stopped using a safe word."
On CSI: Miami, an undercover agent indirectly causes the death of a woman he was using to collect evidence on a drug dealer when he ignores her (coded) call for help.
In the original CSI, Grissom and Lady Heather are having an academic discussion the concept of the safe word in BSDM context, (in a scene rife with Unresolved Sexual Tension), specifically the idea that the dominant must respect requests to stop. Several episodes later the discussion receives a dramatic Call Back - Lady Heather has caught and chained the man who tortured and killed her daughter and, in a rage, is whipping him bloody, and only stops when Grissom invokes the rule and forces her to acknowledge it.
In one of the first "I'm Carol" sketches on Saturday Night Live, Carol was at a BDSM party where they went through everybody's safe words twice because Carol couldn't remember what they were. Script here.
- Another SNL BDSM sketch, involving Kristin Wiig as politician Nancy Pelosi, ended with the sound of a belt sander as a dude is screaming. She runs off stage yelling the safe word. For the record, it was "Palomino".
In White Collar, Neal has to go undercover with minimal surveillance (GPS and audio) to determine whether his target has some stolen bonds. His code for the FBI to move in is "long flight".
- Peter and Elizabeth have "azaleas," which lets Elizabeth know if Peter's all right if he's in a potentially dangerous situation. Mozzie's is "bread basket".
A similar thing in an episode of Mad About You, when Jamie ropes Paul into making a new "I ♥ NY" commercial. They talked about having a code word, like "paramecium" or "Estes Kefauver", but never actually settled on one. Then they were on the set, and by now it's way too late to back out:

Paul: Honey? Paramecium! Big ones!

On The Drew Carey Show, when Drew & Co. went to visit his creepy stalker (but discovered that the stalker had modeled his house, and his life, after Drew's) the signal was for someone to say he was tired. After the stalker takes them prisoner, Oswald says, "Boy, I sure am sleepy," as if that would get them out of their predicament.
In The Pretender, a simulation could be interrupted with the safe word "Refuge".
The title character of Castle has the safe word "apples", used in two episodes so far. There are also hints that Beckett has a dominant streak.
In Gene Simmons Family Jewels, Gene had to experience some domination and had the safe word, "ice cream".
Non-sexual example from Sabrina the Teenage Witch. Sabrina and her friends are going to a party, and are afraid it will be boring. They agree to use "Kazoo" as code for "Get me out of here!"
Important Things with Demetri Martin had a series of skits about a BSDM couple that had chosen unfortunate safewords, one of which was "the correct pronunciation of Gnocchi."
On Desperate Housewives when Rex is teaching Bree the basics of domination, she immediately requests they change the safeword, as it happens to be the city her aunts lives in and doesn't want to think of her aunt during sex.
Barney and Robin from How I Met Your Mother use "flugelhorn."
In one Law and Order SVU episode, the detectives interview a woman who sold the Victim of the Week some BDSM sex toys as they investigate whether the death might have been caused by a sexual accident rather than murder. She's upset at hearing about the victim's death, explaining that she taught her about code words and safety. (Minutes later, the detectives find half a million dollar's worth of diamonds inside the handle of an unused whip, and the investigation is redirected to high finance crimes.)
Discussed on Buffy the Vampire Slayer. After Angel saves Xander from Faith raping and killing him he brings up what she was doing. His dubious reply to them just having fun was Xander forgetting the safe word.

Video Games

Saints Row the Third has a brothel named Safe Word, a club for those with BDSM tastes. Kinzie mentions hers is teacup and she is quickly spirited away before she can reveal more squicky bits of information.

Web Comics

Prominently referenced and sometimes used in Collar 6, seeing how it is all about BDSM. Laura's safe word, for example, is "halo".
Used in this Unshelved strip (involving yoga).
In Shiniez, Ally's safeword, which both Lisa and Anne use when they're with her, is "sunstone." Alan's safeword, which Anne uses, is a three-note hum that can be used through a gag.
xkcd teaches us chemists have the worst safewords.
SMBC teaches us that introducing safewords in nonsexual contexts is a fun way of creeping people out.

Web Original

I'm a Marvel And I'm a DC: Apparently, Gobby's safe word is "pumpkin."
In Tales of MU, Mackenzie's safe word is "basement".

Western Animation

In Family Guy, Peter and Lois use "banana" as the safe word when they do BDSM.
In Team America: World Police, our hero has a safe word when playing with the terrorists, so that his friends will know when to pull him out. Because of the kind of movie this is, of course it doesn't work!
- Even better, it's less a safe word and more a "Safe Wild-Waving-Thrashing Gesture". Which makes it even more hilarious when it doesn't work, despite the sign being really, incredibly obvious.
In the Superjail episode "Cold-Blooded", Jared is sent into the prison undercover (against his will). The warden tells him that if things get rough, he can just say the safe word and Jailbot will get him out of there. Hearing that the safe word is "Baby make weewee," Jared objects that it isn't a word, much to the Warden's annoyance. He then uses it almost immediately, but since the twins messed up Jailbot's circuitry, he's out of luck.
In one Rick and Morty, Healthy Morty (as in Morty after being cleansed of all negativity, causing him to develop a Lack of Empathy) and his girlfriend use "sea cucumber". She's screaming it in The Stinger when finally released from the detoxifier chamber, which she had mistaken for some sort of BDSM dungeon.

Real Life

Most full-contact LARP groups have some sort of safe word that is usually relayed until everybody within earshot will immediately drop any weapons and listen for the coordinator. This is used in the case of injury, immediate danger, or urgent group wide announcements.
On shooting ranges, "Cease Fire!" is the universal safeword, meaning that everybody unloads, disarms, and puts the guns down.
- Unless unloading would be unsafe, in which case the counter-safeword "Loaded!" is used. The range officials will decide when it's safe for the person who calls that to take the shot. Then that person puts the rifle down.
Martial Arts that involve grappling and/or partner drill will have usually something along these lines, with "tapping out" being the most common.
The Society for Creative Anachronism uses "Hold!" as a global safeword, basically indicating that everyone should freeze.
When Christopher Hitchens agreed to be waterboarded for an article in Vanity Fair, he was given not only a safeword ("red"), but two metal objects he could drop as an additional nonvocal signal. He used it.

"We're going to place metal objects in each of your hands. These objects are to be released if you experience unbearable stress. As soon as you release one or both, this exercise and demonstration will end immediately."