It means a die as in a gaming piece. In Classical Latin, the word can refer to the gaming pieces used in dice games, of which there were two types, or a game of chance more generally, or a chancy venture by metaphorical extension. The historian Suetonius reports that when Caesar finally decided to cross the Rubicon River (implicitly rebelling), he said, "Iacta alea esto!" which roughly translates, "Let the die be cast!"

Source: Charlton T. Lewis and Charles Short, A Latin Dictionary (Oxford: Clarendon, 1879)