A universe vaguely similar to our own, but different in one or more fundamental ways. For instance, one alternate universe might exist where the dinosaurs never died out, a second might have gravity pulling at half the force it does in our universe, and a third might be where Kato Kaelin became a popular endorsement for Gatorade.

Alternate universes are popular among science fiction and comic book writers, who frequently use them as plot devices when fanboys want to see a good character acting evil or vice versa. See also: "What If?"