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?"