A writing under seal, by which a person binds himself, his heirs, executors, and administrators, to pay a certain sum on or before a future day appointed. This is a single bond. But usually a condition is added, that, if the obligor shall do a certain act, appear at a certain place, conform to certain rules, faithfully perform certain duties, or pay a certain sum of money, on or before a time specified, the obligation shall be void; otherwise it shall remain in full force. If the condition is not performed, the bond becomes forfeited, and the obligor and his heirs are liable to the payment of the whole sum.