Fanservice is material included to please fans rather than to serve the plot. The term covers two senses: general audience-pleasing extras like cameos and callbacks, and, in its most common usage, sexualized presentation of characters.
Fanservice is an umbrella with a wide canopy. At its broadest it means anything included to delight the existing audience rather than to advance the story: a returning character's cameo, a callback pose, a mecha variant drawn for the model builders, a concert scene staged for the song. In that sense every long-running franchise runs on fanservice and always has.
In common usage the word narrows, most often to sexualized content: costumes, camera angles, and scenarios designed to titillate. That usage dominates discussion, and it is the one this entry's second sense records. Criticism of fanservice in that sense is a normal part of reviewing, argued on craft and taste like anything else.
This site's rules draw a hard perimeter around the term's use in content. Fanservice appears only as a defined concept, as here; it is never applied to underage characters, never treated as a selling point, and never used as quiz material. A glossary can describe the word precisely while the site itself simply declines the practice, which is the arrangement this vertical chose.
In use"A veteran character's surprise cameo in the finale is fanservice in the broad sense: pure reward for long-time viewers."