Countersign

Part of speech: verb (v.)

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.

Part of speech: adjective (a.)

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

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