Reader Insert: A story, usually fanfiction, involving putting the reader into the canon fandom universe.
Reader x Character: A reader insert fanfiction where the character is shipped with another character from the story
RLOTP: A fangirl's real-life OTP (see also: BROTP, NOTP, OTP)
Roleplay (Rp): (v.) To act and speak as if you are the character you're portraying. To roleplay is as much about what not to say as what TO say. It means to keep your speech in the context of the setting in which your character (or avatar) exists. Context can be defined as both time and place.
Rarepair: A Ship or pairing that not many people ship or is very rare to find fanfiction and fanart of. Typically happens within small fandoms or with minor characters.