The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Mil.) A private signal, word, or phrase, which must be
      given in order to pass a sentry; a watchword.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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