To sign on the opposite side of (an instrument or writing); hence, to sign in addition to the signature of a principal or superior, in order to attest the authenticity of a writing.

The signature of a secretary or other officer to a writing signed by a principal or superior, to attest its authenticity.

a second confirming signature endorsing a document already signed

A private signal, word, or phrase, which must be given in order to pass a sentry; a watchword.

a secret word or phrase known only to a restricted group; "he forgot the password"

Countersign is a military term for a sign, word, or any other signal previously agreed upon and required to be exchanged between a sentry or guard and anybody approaching his or her post. The term usually encompasses both the sign given by the approaching party as well as the sentry's reply. However, in some armies, the countersign is strictly the reply of the sentry to the password given by the person approaching.