The Collaborative International Dictionary of English v.0.48:

Proposal \Pro*pos"al\, n. [From Propose.]
   1. That which is proposed, or propounded for consideration or
      acceptance; a scheme or design; terms or conditions
      proposed; offer; as, to make proposals for a treaty of
      peace; to offer proposals for erecting a building; to make
      proposals of marriage. "To put forth proposals for a
      book." --Macaulay.
      [1913 Webster]

   2. (Law) The offer by a party of what he has in view as to an
      intended business transaction, which, with acceptance,
      constitutes a contract.
      [1913 Webster]

   Syn: Proffer; tender; overture. See Proposition.
        [1913 Webster]

WordNet (r) 3.0 (2006):

proposal
    n 1: something proposed (such as a plan or assumption)
    2: an offer of marriage [syn: marriage proposal, proposal of
       marriage, marriage offer, proposal]
    3: the act of making a proposal; "they listened to her proposal"
       [syn: proposal, proposition]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "proposal":
   admonition, advice, advising, advocacy, aim, ambition, animus,
   aspiration, bid, briefing, caution, caveat, consultation, council,
   counsel, desideration, desideratum, design, designation, desire,
   determination, direction, draft, effect, engagement, exhortation,
   expostulation, fixed purpose, function, game plan, guidance,
   hortation, idea, improper suggestion, indecent proposal, instance,
   instruction, intendment, intent, intention, invitation, layout,
   marriage proposal, meaning, mind, monition, motion, motive, naming,
   nisus, nomination, offer, offer of marriage, opinion, outline,
   parley, pass, plan, point, presentation, proffer, project,
   projection, proposition, prospectus, purpose, recommendation,
   remonstrance, request, resolution, resolve, sake, scenario, scheme,
   sexual advance, striving, study, suggestion, tender, thought, view,
   warning, will



Bouvier's Law Dictionary, Revised 6th Ed (1856):

PROPOSAL. An offer for consideration or acceptance.
     2. It is a general rule that a proposal offered to another for
acceptance may be withdrawn at any time before it is accepted, provided that
notice of the withdrawal be given to the party to whom it was made. A bid
(q.v.) may be withdrawn at any time before acceptance; and a proposal by
letter may be withdrawn at any time before, acceptance 1 Pick. 278; and, if
accepted, it must be, in the very terms offered. 3 Wheat. 225. Vide Bid;
Correspondence; Letter; Offer.