For questions about content on television, in order to distinguish it from other types of story. Do NOT use this tag for every question which happens to be about a TV show: it is normally used only on story-ID questions and in cases where it would otherwise be unclear whether the question was about a television show or (e.g.) a film.
Science fiction shows have been on television almost since the invention of broadcast television. The first known example broadcast was R.U.R. (the play), broadcast in the United Kingdom in 1938. Captain Video and His Video Rangers was the first example in the United States, debuting in 1949. This was also potentially the first example of a new property created originally for television.
Over the decades that followed many classic science fiction properties originated as television series, including Star Trek, The Twilight Zone, Doctor Who and Red Dwarf. Fantasy shows have not been quite as well represented.
In recent years the number of science fiction and fantasy shows have exploded as production costs have decreased. Historically this had been the Achilles heel of these shows, as they often had demanding set, prop and special effects requirements.
Television, as it relates to questions here, can be used for either television series, miniseries, or one-off events.