Source : Webster's Revised Unabridged Dictionary (1913)

Countersign \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.

Countersign \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.

Source : WordNet®

countersign
     n 1: a secret word or phrase known only to a restricted group;
          "he forgot the password" [syn: {password}, {watchword},
          {word}, {parole}]
     2: a second confirming signature endorsing a document already
        signed [syn: {countersignature}]
     v : add one's signature to after another's to attest
         authenticity; "You must countersign on this line of the
         contract"