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

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "briefing":
   account, acquaintance, admonition, advice, advising, advocacy,
   airmanship, announcement, appointment, arrangement, basic training,
   bespeaking, blue book, booking, brief, bulletin, caution, caveat,
   clearing the decks, communication, communique, consultation,
   council, counsel, data, datum, direction, directions, directory,
   dispatch, employment, engagement, enlightenment, equipment,
   evidence, exhortation, expostulation, facts, factual information,
   familiarization, final instructions, fixing, flight plan,
   flying lessons, foundation, gen, general information, groundwork,
   guidance, guidebook, handout, hard information, hiring, hortation,
   idea, incidental information, info, information, instruction,
   instructions, intelligence, knowledge, light, makeready,
   making ready, manufacture, mention, message, mobilization,
   monition, notice, notification, opinion, orders, parley,
   pilot training, pilotship, planning, prearrangement, preengagement,
   preliminaries, preliminary, preliminary act, preliminary step,
   prep, preparation, preparatory study, preparing, prepping,
   prerequisite, presentation, pretreatment, processing,
   promotional material, proof, propaedeutic, proposal, provision,
   publication, publicity, readying, recommendation, release,
   remonstrance, report, reservation, retainment, rundown, sidelight,
   spadework, statement, suggestion, taking on, the dope, the goods,
   the know, the scoop, thought, training, transmission, treatment,
   trial, tryout, warm-up, warning, washout, white book, white paper,
   word