The Collaborative International Dictionary of English v.0.48:

briefing \briefing\ n.
   1. giving detailed instructions, as for a military operation.
      [WordNet 1.5]

   2. a detailed set of instructions before an operation, given
      to those who are to execute it.
      [PJC]

   3. a summary report of an operation or event after it has
      occurred, by a participant or observer.
      [PJC]

WordNet (r) 3.0 (2006):

briefing
    n 1: detailed instructions, as for a military operation