The term originally referred to scenes which were gloriously animated just for fan appeal. It has come to refer to scenes with scantily (or not at all) dressed characters.
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.
Ecchi, Hentai - These are different terms for varying degrees of sexual content. Fan Service usually refers to non-explicit titillation (i.e. panty shots, jiggle, skimpy bathing suits). Ecchi is more explicit and pretty much implies nudity. Hentai is actual porn. Ecchi dialogue is off-color without outright using explicitly crude words. The words "ecchi" and "hentai" are also used to refer to mild or big-league perverts respectively. ("Ecchi" is the Japanized pronunciation of the letter "H", which is the first letter in the Romaji spelling of "Hentai".)
fulfilling the fantasies a show's fans have for its characters. In the case of fanboys, this frequently involves t&a shots. In the case of fangirls, this can include bishounen in eye-candy shots. In both cases it can include homoerotic scenes. The term is not used exclusively for anime/manga.
Titillating elements, like toplessness or panty shots, that aren't strictly necessary to the story, but are gratuitously included anyway for the benefit of young male readers.
gratuitous shots (i.e. ones that have no purpose in the storyline) added to animé or manga purely to please fans. Fan service usually comes in the form of close-ups of certain parts of a female character's anatomy or brief flashes of them in a state of undress, although in certain cases the term can also cover cameo appearances by animal mascots and the like (the obligatory shot of Ryo-Ohki sitting on Sasami's head going "Miya!" is a textbook example).
At its most basic, fan service is the gratuitous display of (usually) female characters in such a way as to show their panties, other undergarments, or bare skin. However, frontal (and usually undetailed) nudity is pretty much the limit.
This is a general term for any scene that is not integral to the plot line, but included only to excite or titillate a viewer. This can range from cleavage shots to gratuitous battle explosions. Also simply known as "service."
Term used for anime where there are scenes with nudity/revealing clothing of an anime character intended to get the fans excited or 'reward' them (hence 'service') for indulging in the release.
A term used for moments in anime or manga where a character flashes a bit of flesh or underwear to titillate the viewer. This can be anything from full frontal nudity to a character in their swimming costume, and is often something as tame as a quick flash of their pants. Compromising situations are often crowbarred into anime and manga simply for this purpose, which is why walking in on female characters when they are bathing is such a common occurrence...
Usually used to mean, basically, stuff to titilate adolescent male fans. It mostly includes those shower/bath scenes that usually have little to do with the story but seem to be generally appreciated by the fans... or at least the teenaged male ones. Also can be used more generally (if less frequently) to refer to almost anything unrelated to the story but included to please fans--fancy outfits, flashy mecha, a cute animal, that sort of thing.
Contains material to titillate adolescent male fans. The most common form being pose shots of female characters. Examples include shower or bath scenes that usually have little to do with the advancement of the story but seem to be generally appreciated by the fans.
Ever wonder why sometimes in an anime, the characters will go to a spa and get naked or take a lengthy shower and it does nothing to do with the plot whatsoever? That's fan service for you folks, put in a show purely for the purpose of pleasing fans.
a euphemism for gratuitious female nudity, ranging from brief breast or panty shots to full frontal nudity; often used solely to attract hentai viewers to anime with lower plot quality