A gatecrasher is someone who enters a venue or attends an event without invitation or proper credentials. Examples include that guy eating all the hors d'oeuvres at your party who you didn't invite, and that girl that sneaks into the concert while you paid Ticketmaster some ludicrous service fees.

The word is sometimes seen hyphenated, but has been in sufficient common use since the 1920's to warrant promotion to a non-hyphenated compound noun.

Gatecrasher is also a character in the Marvel Comics universe. She is a large, blue humanoid being, who is either an alien from this universe or from another dimension altogether. She was the leader of a bounty hunter group known as the Technet, who made appearances primarily in the USA in Excalibur comics, but have appeared in UK comics as well. She is sufficiently strong for her size and wields wrist blasters. She always has a green creature named Yap on her back, who provides information to her and has teleportation powers.