An absentee is someone who doesn't show up when she's supposed to. A high school student who's an absentee too often may end up failing some classes.
If the star player of your basketball team doesn't show up for the big game, she's an absentee. Likewise, if you skip work to go to the beach on a beautiful day, your boss will consider you an absentee. If you fill out an absentee ballot before an election, you're using a special form for voters who won't be physically present at the polls. The Latin root is absentem, "be away from."