firefly, Firefly, n., p.n. 1. also known as lightning bugs, are nocturnal, luminous beetles, prevalent east of the Mississippi and Asia. 2. A series created by Joss Wheadon first aired on Fox Network in the fall season of 2002-03. Cancelled after a single half-season, Firefly quickly became a cult classic, spawning a number of fansites on the Internet and a major motion picture in 2005 named [1].

Addendum (def #2): Joss Wheadon is also famous for creating Buffy the Vampire Slayer and its spinoff Angel. Firefly was unique for taking a science fiction world (based in the year 2517, after the Earth was used up and mankind was forced to expand) and giving it a western flair (many of the so-called Outer Rim planets were new or barely scraping by, their residents reverting to low-tech options in order to survive). Starring Nathan Fillion, Gina Torres, Adam Tudyk, Adam Baldwin, and a host of relatively new actors/actresses, Fox canceled it due to “lack of ratings”, though by the end the show had gained up to 9 million viewers. Some fans blamed Fox for lack of advertising, and also for showing the episodes out of order, thereby scrambling the plot and confusing the viewers. Fox's cancellation lead to an antagonistic attitude from Firefly fans (who often refer to themselves as Browncoats), who lead a number of unsuccessful letter-writing campaigns and internet petitions to reinstate the show. After cancellation, however, word of mouth helped to push the later DVD collecting the series to heights enough for Universal to consider making it into a movie. Due to Serenity's modest showing at the box office, there is a slim chance the movies will continue (there was a trilogy planned), but fans are pushing sales of the DVD, in a mirror to the strong showing for the series' discs. There is slim hope that Universal, the Sci-Fi Channel (who picked up the syndication rights), or Paramount (rumors spread after the cast was seen leaving the Paramount studios that the new CW Network was going to pick up the series) will produce a new series. And now you know... the rest of the story.
Addendum #2: Firefly's influence on geek culture in the subsequent years is much like any cult classic movie; the slang from the movie has been adapted by many fans, including the Chinese phrases peppered into the speech. (In Firefly, the main culture is a mix of Anglo and Chinese, hence the use of the language in normal speech.)
Addendum #3: Margaret Weis Productions (the current producers of Dragonlance material for d20) produced Serenity RPG in conjunction with the movie's release in the summer of 2005.