A parallel existence in which events may unfold differently and particles are antimatter and so forth. A subset of the multiverse.
Amply illustrated in this Youtube conversation.
Dude 1: I shudder to think that there may be an alternate universe out there in which McCain-Palin won the elections! Wait, she could be PRESIDENT in this one!
Dude 2: There is such a universe. In that universe Gore and Lieberman won. Gore was killed by an Iraqi-Iranian-Pakistani Terrorist while he was looking out a window in the Twin Towers. President Lieberman made a tearful speech declaring Gore a martyr. He the declared World War on Islam and bombed Egypt.