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]