A latin conjunction meaning exclusive or (XOR for CS people). In other words, "A aut B" means that either A is true or B is true, but not both.

For example, "aut vincere, aut mori" (victory XOR death) or "aut Caesar, aut nihil," a motto attributed to Cesare Borgia and meaning "I will either be Caesar or I will be nothing." The historical reference is obviously Julius Caesar.