The twelve men chosen by Jesus to follow him and to spread the gospel after his death. They included Peter, James, John, Thomas, Matthew, and Judas Iscariot (who was later replaced). Paul, even though he was not one of the Twelve, is generally considered an apostle because of his crucial role in the spread of Christianity.
Note: In general usage, an apostle is someone who preaches or promotes a cause, particularly a religious one. The term also connotes a disciple.