Literally, to sign on the opposite side of an instrument or writing; hence, to sign, as secretary or other subordinate officer, a writing signed by a principal or superior, to attest the authenticity of the writing. Thus charters signed by a king are countersigned by a secretary. Bank notes signed by the president are countersigned by the cashier.

A private signal, word or phrase, given to soldiers on guard, with orders to let no man pass unless he first names that sign; a military watchword. Advance and give the countersign.