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1.
In general, fan service refers to scenes designed to excite or titillate the viewer. This can include scantily-clad outfits, cleavage shots, panty shots, nude scenes (shower scenes especially), etc. Some broader definitions also include things like cool mecha, big explosions, battle scenes, etc. Basically, if it has little plot-redeaming value, but makes the viewer sit up and take notice, it's probably fan service in one form or another.
In Evangelion, the episode preview for "Next time, service, service!"
by MoonKnight March 28, 2003
 
2.
Usually seen in reference to Japanese popculture. When someone (i.e. manga/anime character, music artist) flashes the general public and/or whenever two or more famous people engage in any action which could be perceived as sexual in nature just to stand out or please (excite, shock, etc.) their fans. Many bandmembers have been known to kiss, lick, rub, and hump against other bandmembers while performing or while in public. Usually, this is purely platonic, but not always.
Person #1: Did you see the ~ *insert band name here* performed at their last concert?
Person #2: Where the vocalist licked the bassist's neck, and later on the bassist humped the vocalist?
Person #1: Yeah, wasn't it great?!
by Lady Arsenic July 28, 2004
 
3.
As stated above, "fanservice" came from Japanese anime, film, and music culture. Its definition is spreading to include any unnecessary visuals included simply because the fans, presumably, want to see them. Gratuitous nudity, male or female, but also shots of cute mascot animals (e.g., Chuchu the mouse in "Utena").
Jay (watching a Chinese wuxia film): They just spent ten minutes showing us Alec Su taking a bath.

Shiu: Fanservice ...
by Jay Young May 10, 2005
 
4.
In anime, fanservice stands for scenes which are not relevant to the overall plot at all, but merely are included to give the viewers what they presumably want to see. This generally includes things like panty shots, bouncing boobs, shower scenes, skirts which barely cover the crotch area, but can also be things like incredibly overexaggerated explosions, giant robots or supercute animals.
Do an image search for "Agent Aika" and you will see. That series basically is ONLY fanservice without a plot.
by jbee2006 May 29, 2006
 
5.
Catering to fans' base interests instead of to the integrity of the artistic work as a whole.
In the middle of the film, the director included two minutes of fanservice, thus providing many twelve year-olds with masturbatory material.
by the letter d December 21, 2008
 
6.
In terms of Anime, the least extreme form of nudity or sexually based angles. This may include panty shots, minor nudity, underwear shots, bouncing breasts and other such things added solely for comic effect or pleasuring perverted fans.

A less severe form of Ecchi or Hentai.

mostly found in romantic comedies and comedy.
1. That's not hentai! Ecchi maybe, or even fanservice.
by Dukeofyo90 September 22, 2005
 
7.
Generally any situation or scene in a movie, cartoon, comic, or even real life in which a scene is done specifically to cater to what the fans want to see, divorced from a plot or any other non-catering need. Often mistaken as invented by Japanese anime, the truth of the matter is that Hollywood movies and American TV shows in particular had been using this concept for years, if not decades, before anime ever even existed; however, the coined word itself DID start within anime fandoms to reference scenes of near-nudity that held no part to the overall plot of the episode/movie/etc.
The first film to involve nudity in the US was 1959, whereas the advent of the modern style and tradition of anime was in the late 60's, hence fanservice existed prior to anime.
by Creed of Heresy January 28, 2011