Fanfiction: "'Fan fiction', broadly speaking, is any kind of written creativity that is based on an identifiable segment of popular culture, such as a television show, and is not produced as 'professional' writing."
Rebecca Tushnet, "Legal Fictions: Copyright, Fan Fiction, and a New Common Law", Loyola of Los Angeles Entertainment Law Journal, Volume 17, 1997. http://patriot.net/~rjpugh/writer/tushnet.html

Slash: "'Slashing' is the process of reading, critiquing, and rewriting media 'texts' -- television shows and movies -- to reveal and explore homoerotic relationships between male characters. Such writing is called 'slash' because of the '/' that is put between the characters' or program's initials as a code to notify the reader that a story contains homosexual erotica."
Patricia Scheiern Lewis, "Pass the Crisco, Spock", LIVEculture, Volume 1, Number 1. http://www.ilt.columbia.edu/projects/live_culture/lc1/articles/slash.html

Mary Sue: "'Mary Sue' is a label given to an original character that is an unrealistic projection of the author, or someone close to the author. Mary Sues tend to be very good at everything they do, and they do everything as well as, or better than, the established characters. They show up in almost every scene, do whatever it is the author wants them to do (often to the appreciation and awe of the regular characters), then humbly vanish."
Richard Pugh, "Look Out! It's Mary Sue!" http://patriot.net/~rjpugh/writer/marysue.html

PWP: Acryonym for either "Porn Without Plot" or "Plot? What Plot?" Generally used to describe a short fic that consists purely of a sex scene.