A recording of laughter which is used by television networks to make viewers of sitcoms believe that the shows they are watching are (A) filmed live and (B) wildly funny. Sometimes called canned laughter.
Recorded laughter isn't used on every comedy show. Even as far back as the 1950s, there were programs that refused to use laugh tracks, or that eventually stopped using them. In its initial seasons, "M*A*S*H," for example, used laugh tracks, but eventually reduced them as the show leaned in harder on its dramatic scenes -- and they never used a laugh track at all during scenes in the operating room. Other shows choose not to use a laugh track, either because they dislike having to tell the audience where the jokes are, because they feel there are enough dramatic moments in the show to make the laugh track obnoxious, or because they feel their shows are pitched to an audience that is mature and intelligent enough not to need a raucous laugh track.
Still, there are lots of shows that love laugh tracks. Even classic comedians like Bob Hope and Milton Berle liked having laugh tracks on their programs -- their job was to make audiences laugh, and fake laughter was better than no laughter at all.
Strangest place for a laugh track: cartoons. Is there an audience somewhere watching those poor animators create a live Scooby Doo cartoon?