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An episode which consists mainly of fragments (clips) of previous episodes, sometimes with voiceover added. Usually has a theme: for example, to highlight a character's development over the years, or show the relationship between characters. Sometimes, however, it won't be shown that the events take place in the past, but they are shown as appearing directly one after another.
Clip shows can be used to stretch the budget — they utilize footage already shot, thus needing only narrative glue money for the episode. In that sense, they are similar to a Bottle Episode. They can also be used to stretch the story — when a show's episode order is suddenly expanded after its early episodes prove popular, but the writers don't have enough story to fill quite that many episodes, clip shows are a popular recourse.
Sometimes regular episodes turn into clip shows when they are localized for other markets, as it's simple and inexpensive to edit in recap segments made up of clips from prior episodes to replace the gaps made when nudity and other risqué content is deleted. (Occasionally this can even happen before an episode is aired in the original market, as was the case with Tengen Toppa Gurren Lagann's infamous Hot Springs Episode.)
One place you might see a clip show is in the second-to-last episode of a season; it's used to conserve the last of the season's budget, so they can blow the wad on a kickass finale.
When a clip show is used to sum up a season or storyline, it is a Recap Episode.
Clip shows were more appreciated by viewers in the days before reruns, syndication, and videotapes/DVDs provided an alternative way for them to revisit the old moments of their shows. There were even theatrical films that served as these, most famously the That's Entertainment! films in the 1970s. In those days, they were less likely to get today's cynical reaction, "They've just done it to save money." They still occasionally emerge in today's TV productions, though mainly in children's programming. In the rare occasion where they manifest in adult scripted programming (such as occurred with Alias) they are usually built into an ongoing story arc. Otherwise, bottle shows are more likely to be produced these days. Due to their low popularity, clip shows are practically more parodied than played straight these days—a very common choice is to have a clip show setup, only for the actual clips to be from stories and events we never actually saw, or be comically off from the ones we did see.
A variant on the clip show are the recap or catch-up specials that often precede new-season premieres for arc-heavy series that serve to help new and returning viewers understand the storyline so far. Shows that have done these sorts of specials in recent years (they are rarely considered regular episodes) have included Farscape, Lost (several), Once Upon a Time, Doctor Who and Game of Thrones. Often, these specials are documentaries, with narration or on-screen introductions by cast members; occasionally, however, the actors appear in character (or, as occurred with the Farscape: Undressed, there was a mixture of both).
Another variant is a clip show using footage not from any one particular series, with voice-over narration. These shows are usually factual, like rolling news, Magazine Shows, or nature documentaries, but purely entertaining shows with this format also exist.
When previous clips of a single character's line or action are played out in rapid succession (such as Homer's "D'oh" sequence in So It's Come To This: A Simpsons Clip Show), that's a Fully Automatic Clip Show.
Compare this to the use of the Magical Security Cam. For instances of a feature-length film essentially being a Clip Show, see Compilation Movie.
