The death sentence for a television show. Usually due to low Ratings, the network decides to stop production of a show. Usually, this also includes ceasing the broadcast of unaired episodes, but some shows are given enough slots to finish their run (that often depends on what time of year the axe falls and what's available to replace it as much as anything else).
Television networks are largely for-profit and funded by advertising. If a show has low viewership, then it is difficult to sell ad time. Shows that are cheaper to produce have a larger profit margin and tend to last longer than big budget shows with the same number of viewers. And occasionally, a show just gets Screwed by the Network and it doesn't matter if the show was profitable, or more often, the show becomes unprofitable thanks to the screwing.
One has to note that a show can't merely get good ratings; it has to get good ratings within a clear Demographic that advertisers can target. In this sense, there are many ways a show with good ratings might get canceled anyway. A show with Multiple Demographic Appeal might appear to have strong ratings, but the actual viewership of each demographic is comparatively low. Many networks gear themselves towards specific demographics with advertising prospects to match and a show with most of its viewership from a Periphery Demographic is considered more trouble than it's worth (the fate of many animated shows tie into this). Lastly, the elderly and southern rural types are considered audiences that simply cannot be reliably targeted and are usually completely ignored when determining ratings. This is why The Beverly Hillbillies was ultimately canceled in spite of being a large ratings draw.
In almost all cases this means that the show is gone forever. In some lucky cases The Resolution Will Not Be Televised, but released on video (or these days, online). In even luckier cases the series will be able to Wrap It Up and a miniseries or movie will resolve the story. Almost unheard of is getting Uncancelled.
If the series creators decide to end the show, the show ends with a Grand Finale and usually isn't labeled as "canceled". Similarly, if a show runs to its scripted end it isn't labeled "canceled".
Some shows are bad enough to be canceled after one episode or even during the first episode. We here at the wiki have dubbed such a series a One-Episode Wonder.
And, of course, this isn't necessarily limited to TV, these days.
MMORPGs also need a moderately large and somewhat stable playerbase to remain viable. For subscription MMOs, this is usually around the 100k mark, for a relatively new entry - for an older game (that's already made its investment back), this can dip as low as 10k, in some cases, especially if it's running in what most gamers call "maintenance mode" - no real updates, just a skeleton crew of interns making sure the servers don't explode and that major exploits get squashed within a few days.
For a list of Cancelled online games, see here.
While the Fan Dumb likes to make proclamations of impending doom on a daily basis, most games that have survived the first year don't particularly have to concern themselves with this, unless it's made public knowledge that the game is in maintenance mode. To date, only a few catastrophic flops have been canceled since the major boom of the MMO genre.