Someone who is not good enough, up to par, achieves satisfactory expectations. Someone who would be the best at what they do if only they are given the chance, which they never do.
Friend 1: Hey! What happened to Cindy? I heard she went home.
Friend 2: Yea, she was only an alternate so they had better people in line before her. So she didn't get the position she wanted and was turned away.
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.