In Episode 11 of Season 1, Jackie (portrayed by Mila Kunis) confused fans of the show, young and old, with her use of a particular slang term. 

In the episode, Jackie is marveling at Eric hanging out with his lab partner, Buddy (played by Joseph Gordon Levitt), who is above his social standing. She then breaks down the hierarchy. "You see, Buddy is in the rich kid clique. They mingle with the jock clique," she explains. "Then you have your smokers and your heads, okay? The rich kids can be smokers and jocks can be heads but jocks cannot be smokers unless they're rich." 

Many Reddit threads have been dedicated to determining which "heads" Jackie was talking about. Redditors argued she meant potheads, noting the creative ways the show kept the teens from ever actually referring to marijuana directly. Another considered it could be a Grateful Dead reference. "I know Grateful Dead fans have been nicknamed 'deadheads' so maybe it could be a reference to that," they suggested. Guess fans will never know for sure, but the theories are just as interesting.