1. Literally, an activity that a student does in addition to their studies (the curriculum). With your grades and extracurricular activities, you'll get into any college you want.

2. slang Sexual activity, often that which is clandestine. If Jen didn't come home last night, I bet she and Bill were preoccupied with some extracurricular activities, if you know what I mean.

3. slang Fighting or arguing that occurs among competitors during a sporting event. It looks like both teams were flagged for some extracurricular activity after that last play.