Annual convention for Comic books, science fiction, fantasy and related popular arts held in San Diego, California every summer.
It is the largest convention of its kind.
Besides exhibitions about comic books and video game companies, there are previews of upcoming movies, panels and seminars with actors and producers of related shows, and workshops with comic book professionals. A lot of visitors wear costumes of their favourite comic book/science fiction/fantasy heroes and there is a costume contest awarding the best ones.
References in the show
The guys appear to go to Comic-Con every year and wear costumes (0108, 0202, 0301). They have the deal that "what ever happens in costumes at Comic-Con, stays at Comic-Con" after Raj had a romantic encounter with Richard the Slave Girl (0202). Sheldon was very upset that he missed Comic-Con in 2009 because of his expedition to the North Pole (0301).
The TBBT cast and crew at Comic-Con
The cast of the show and the two executive producers have appeared at Comic-Con in 2008 and 2009.