A television pilot (also known as a pilot or a pilot episode and sometimes marketed as a tele-movie) is a standalone episode of a television series that is used to sell the show to a television network. At the time of its creation, the pilot is meant to be the testing ground to gauge whether a series will be successful. It is, therefore, a test episode for the intended television series, an early step in the series development, much like pilot studies serve as precursors to the start of larger activity.
In the case of a successful television series, the pilot is commonly the first episode that is aired of the particular series under its own name--the episode that gets the series "off the ground". A "backdoor pilot" is an episode of an existing successful series that heavily features supporting character(s) or guest stars in previously unseen roles. Its purpose is to introduce the characters to an audience before the creators decide on whether or not they intend to pursue a spin-off series with those characters.
Television networks use pilots to determine whether an entertaining concept can be successfully realized and whether the expense of additional episodes is justified. A pilot is best thought of as a prototype of the show that is to follow, because elements often change from pilot to series. Variety estimates that only a little over a quarter of all pilots made for American television proceed to the series stage.