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"

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "countersign":
   ID card, John Hancock, OK, X, accept, accredit, affirm, amen,
   approve, assure, authenticate, authorize, autograph, avouch, badge,
   calligram, calling card, card, certify, christcross, cipher,
   confirm, cosign, countermark, countersignature, counterstamp,
   credentials, cross, device, dog tag, endorse, endorsement,
   give permission, give the go-ahead, give the imprimatur,
   give thumbs up, guarantee, hand, identification,
   identification badge, identification tag, initial, initials,
   letter of introduction, make a promise, mark, mark of signature,
   monogram, notarize, open sesame, pass, pass on, pass upon,
   password, permit, pledge, plight, press card, promise, ratify,
   rubber stamp, sanction, say amen to, seal, second, secret grip,
   serial number, shibboleth, sigil, sign, sign and seal, sign manual,
   signature, signet, subscribe to, subscription, support, swear,
   swear and affirm, swear to, tessera, token, troth, undersign,
   underwrite, validate, visa, vise, visiting card, vouch, vow,
   warrant, watchword, word