The Collaborative International Dictionary of English v.0.48:

Imprimatur \Im`pri*ma"tur\, n. [L., let it be printed.]
   1. (Law) A license to print or publish a book, paper, etc.;
      also, in countries subjected to the censorship of the
      press, approval of that which is published.
      [1913 Webster]

   2. (R. C. Ch.) Permission granted from a designated
      eccliastical authority to publish a book or other
      document; -- required by church law for Catholics,
      especially ecclesiastics, who wish to publish.
      [PJC]

   3. Hence: Official approval for some proposed activity; as, a
      contract this large needs the imprimatur of the legal
      department.
      [PJC]

WordNet (r) 3.0 (2006):

imprimatur
    n 1: formal and explicit approval; "a Democrat usually gets the
         union's endorsement" [syn: sanction, countenance,
         endorsement, indorsement, warrant, imprimatur]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "imprimatur":
   John Hancock, OK, acceptance, affirmance, affirmation, approbation,
   approval, authentication, authorization, building permit,
   certification, confirmation, countersignature, endorsement,
   fishing license, go-ahead, green light, hunting license, license,
   nihil obstat, nod, notarization, okay, permission, permit,
   ratification, rubber stamp, sanction, seal, sigil, signature,
   signet, stamp, stamp of approval, subscription, the nod,
   validation, visa, vise, warrant