Money or a promise that is put down by a defendant to make sure he will come back to court.  For example, bail may be set at $5000 cash.  In that case, the defendant (or more likely someone acting on his behalf) has to deposit $5000 with the court.  If he does not appear for court, he loses the money and the police come to arrest him.  If he comes to court, the bail is returned at the end of the trial.