Exactly What It Says on the Tin, this is when a character first appears in an act of fiction. Please do not list examples here (everyone appeared at some point, after all). This trope belongs on a work's recap or trivia page.
This trope could also apply to a famous structure first appearing as well.
If it's about the first time a trope is used in a series, use that trope.
Compare to Trope Maker and Ur-Example.