Coun`ter*sign" (-s?n`; 277), v. t. [imp. & p. p. Countersigned (-s?nd`); p. pr. & vb. n. Countersigning.] [Counter- + sign: cf. F. contresigner.] 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.

Coun"ter*sign`, a. 1. The signature of a secretary or other officer to a writing signed by a principal or superior, to attest its authenticity.

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