A series finale refers to the last installment of an episodic entertainment series, most often the final episode of a television series. It may also refer to a final theatrical sequel, the last part of a television mini-series, the last installment of a literary series, or any final episode.
Origins in television
Most early television series consisted of stand-alone episodes rather than continuing story arcs, so there was little reason to provide closure at the end of their runs. Early series that had special finale episodes include Howdy Doody in September 1960, Leave It to Beaver in June 1963, and in early 1964, Route 66.
Considered to be "the series finale that invented the modern-day series finale," the 1967 final episode of ABC's The Fugitive, "The Judgment: Part 2", attracted a 72% audience share when broadcast. This remained the highest viewership percentage in U.S. television history until the 1977 finale of the TV mini-series Roots (on the same network) and later the 1980 resolution episode of the internationally prominent "Who Shot J.R.?" cliffhanger of CBS' Dallas.
Notable television series finales
Most watched series finales
The most watched series finale in television history remains the 1983 finale of the CBS war/medical dramedy M*A*S*H, titled "Goodbye, Farewell and Amen". Viewed by 105.9 million viewers and drawing 77% of those watching televisions at the time, the finale of M*A*S*H held the record for most watched telecast of all-time for decades until 2010's Super Bowl XLIV edged it out with 106 million viewers, which coincidentally also aired on CBS. However, M*A*S*H's final episode remains the all-time most watched U.S. television episode (and so far, the only single television episode in U.S. history to be watched by at least 100 million viewers for a single telecast). 
The second-most watched series finale in U.S. television history is the 1993 finale of the witty, character-based NBC comedy Cheers, titled "One for the Road". "One for the Road" was watched by between 93.5 million and 80.4 million viewers (estimates vary) while drawing 64% of TVs turned on at the time. To date, "One for the Road" remains the most watched U.S. TV series finale following the rise of cable television, and in terms of sheer viewership numbers for non-sports programming, sits second only to the aforementioned finale of M*A*S*H 
With only slightly fewer viewers than the series finale of Cheers is the finale of its one-time follow-up on NBC's "Must See TV" Thursday night line-up, the absurdist NBC comedy Seinfeld. The fourth most watched U.S. TV series finale in television history, Seinfeld's controversial 1998 episode "The Finale" was watched by 76.3 million people, drawing 67% of all televisions turned on at the time - as the New York Times put it, "grazing Super Bowl country" in terms of viewership.
With the shift away from network television viewing toward cable television viewing (and later, internet use) that occurred during the decade between the finales of M*A*S*H (1983) and Cheers (1993) - and continued unabated until and beyond the finale of Seinfeld (1998) - it remains debatable which of these three "event" series finales accomplished the most impressive viewership numbers. Moreover, a large gap in viewership numbers exists between the Super Bowl-sized audiences of the M*A*S*H, Cheers and Seinfeld finales, and the fifth and sixth most watched series finales in television history - respectively, those of the comedy Friends (2004, NBC, 65.9 million viewers) and the detective procedural Magnum, P.I. (1988, CBS, 50.7 million viewers). The Friends finale's viewership numbers dwarf those of all finales since the start of the new millennium and seem particularly impressive in light of the increased media options since the 1990s "event" finales of Cheers (1993) and Seinfeld (1998).
No matter how critically lauded during their respective runs, relatively few popular television series finales end up pleasing critics and audiences universally and/or escaping controversy - with the finale of the comedies Seinfeld ("The Finale", 1998), How I Met Your Mother ("Last Forever," 2014), Two and a Half Men ("Of Course He's Dead", 2015), – and the mob drama The Sopranos ("Made in America", 2007) being some of the better known examples of this trend. For example, The Sopranos finale caused millions of viewers to temporarily believe they had lost cable service due to an abrupt blackout.
Several iconic television series' finales did, however, manage to produce episodes that lived up to critics' and audiences' expectations; for example, the twist endings that concluded both the Newhart and St. Elsewhere finales, the mixture of comedy and resonance that wrapped up both The Mary Tyler Moore Show and Cheers, and the redemption story that completed the arc of The Fugitive. Several series finales have won awards for their excellence, including those of Star Trek: The Next Generation and Lost.
Recent series finales receiving high praise include the Breaking Bad episode "Felina" (2013) and earlier in the millennium, the 2005 series finale of Six Feet Under, "Everyone's Waiting", which TV Guide ranked #22 on their list of "TV's Top 100 Episodes of All Time".
Television series finales frequently feature fundamental deviations from the central plot line, such as the resolution of a central mystery or problem, (e.g. Dallas, Two and a Half Men, Full House) the separation or return of a major character (e.g. Cheers, That '70s Show, The Office) or an event signifying the end of an era, such as a change to primary setting for the series (e.g. The Mary Tyler Moore Show, Boy Meets World, Martin.)
Series finales will sometimes include clips or characters from the series' past (e.g. Seinfeld, Six Feet Under, Martin, Star Trek: The Next Generation.), and the ending moments of the episode often takes place in the show's primary setting.
Premature series finales
In some cases a TV series finale proves premature, as was the case with Here's Lucy, Charmed, 7th Heaven, and Babylon 5, to name but a few. Some shows that have constantly been in danger of cancellation wrote every season finale with the idea that the episode would serve as a quality series finale if the network decided not to bring it back; in recent years from NBC's Thursday night comedy lineup, "Parks and Recreation" used this formula for the season finales for Seasons 3-6, before getting a renewal for a 7th and final season where the series finale was planned in advance, and "Community" wrote its 5th season finale with the notion that whether the show found new life elsewhere or not, it would definitely not be returning to the network (while NBC did indeed cancel the show, it was renewed for a 6th season by Yahoo Screen, where the season, and sure enough, series finale was once again scripted as a potential last episode ever; the final image is that of text reading "#andamovie", implying that the series would wrap up entirely with a movie, which has yet to be made).
The medical comedy Scrubs aired its two-part finale episode billed simply as a "My Finale" in May 2009 as the show's renewal or cancellation had not been decided as of its airing, and so it was not known whether the episode would conclude just the season or the entire series; Scrubs would eventually be renewed for one additional season, which became a spin-off series titled Scrubs: Med School.
The cartoon Futurama has had four designated series finales, due to the recurringly uncertain future of the series. "The Devil's Hands Are Idle Playthings", "Into the Wild Green Yonder (Part 4)", "Overclockwise", and "Meanwhile" have all been written to serve as a final episode for the show.
The series finale of The Lucy-Desi Comedy Hour (itself an epilogue to I Love Lucy) was unintentionally fitting: stars Lucille Ball and Desi Arnaz were about to divorce and end the show, a fact that the show's guest star for what would be the final program, Edie Adams, did not know when she chose the song she would sing on the program. Prophetically, the song was named "That's All." The series also ended with Lucy and Ricky making up and kissing, while in reality Ball and Arnaz would not (the two would eventually reconcile later in life, although both would go on to marry other people).
The afore-mentioned Magnum, P.I. had a premature series finale, as well. At the end of the seventh season, protagonist Thomas Magnum was to be killed off, which was intended to end the series. The final episode of the season, "Limbo", after seeing Magnum wander around as a ghost for nearly the entire run-time, closes with him appearing to walk off into heaven. However, following outcry from fans, who demanded a more satisfactory conclusion, an eighth, final season was produced, to bring Magnum "back to life", and to round the series off. A number of other episodes also make reference to supernatural occurrences and the seeming existence of ghosts.
The Showtime series, Californication, was designed from start to make any season finale work as a series finale, in case of early cancelling the show. It is seen most primarily at the end of the first and fourth season.
Finales launching spinoffs
Series finales are sometimes used as a platform to launch spinoff series. Two well-known examples include the The Andy Griffith Show's series finale, which launched the spinoff Mayberry RFD and The Practice's series finale, and much of its final season, was used as a launching pad for Boston Legal, starring James Spader and William Shatner. Three's Company launched the spinoffs The Ropers and Three's A Crowd
Some planned spin-offs that influenced series finales, however, never materialized, as in the case of the proposed Laverne & Shirley spin-off for Carmine that never came into fruition or the Star Trek: The Original Series episode Assignment: Earth, which was intended as a backdoor pilot for a TV series of the same name which did not materialize.
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