The Collaborative International Dictionary of English v.0.48:

Convention \Con*ven"tion\, n. [L. conventio: cf. F. convention.
   See Convene, v. i.]
   1. The act of coming together; the state of being together;
      union; coalition.
      [1913 Webster]

            The conventions or associations of several particles
            of matter into bodies of any certain denomination.
                                                  --Boyle.
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   2. General agreement or concurrence; arbitrary custom; usage;
      conventionality.
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            There are thousands now
            Such women, but convention beats them down.
                                                  --Tennyson.
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   3. A meeting or an assembly of persons, esp. of delegates or
      representatives, to accomplish some specific object, --
      civil, social, political, or ecclesiastical.
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            He set himself to the making of good laws in a grand
            convention of his nobles.             --Sir R.
                                                  Baker.
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            A convention of delegates from all the States, to
            meet in Philadelphia, for the sole and express
            purpose of reserving the federal system, and
            correcting its defects.               --W. Irving.
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   4. (Eng. Hist) An extraordinary assembly of the parkiament or
      estates of the realm, held without the king's writ, -- as
      the assembly which restored Charles II. to the throne, and
      that which declared the throne to be abdicated by James
      II.
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            Our gratitude is due . . . to the Long Parliament,
            to the Convention, and to William of Orange.
                                                  --Macaulay.
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   5. An agreement or contract less formal than, or preliminary
      to, a treaty; an informal compact, as between commanders
      of armies in respect to suspension of hostilities, or
      between states; also, a formal agreement between
      governments or sovereign powers; as, a postal convention
      between two governments.
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            This convention, I think from my soul, is nothing
            but a stipulation for national ignominy; a truce
            without a suspension of hostilities.  --Ld. Chatham.
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            The convention with the State of Georgia has been
            ratified by their Legislature.        --T.
                                                  Jefferson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

convention
    n 1: a large formal assembly; "political convention"
    2: something regarded as a normative example; "the convention of
       not naming the main character"; "violence is the rule not the
       exception"; "his formula for impressing visitors" [syn:
       convention, normal, pattern, rule, formula]
    3: (diplomacy) an international agreement
    4: orthodoxy as a consequence of being conventional [syn:
       conventionality, convention, conventionalism] [ant:
       unconventionality]
    5: the act of convening [syn: convention, convening]

Moby Thesaurus II by Grady Ward, 1.0:

248 Moby Thesaurus words for "convention":
   Council of Nicaea, Council of Trent, Lateran Council, NATO, SEATO,
   Vatican Council, accord, agreement, alliance, amenities,
   arrangement, assemblee, assembly, assignation, at home, audience,
   ball, bargain, bargaining, bargaining session, bienseance,
   binding agreement, bon ton, bond, brawl, canon, capitulation,
   cartel, caucus, chapter, civilities, civility, classis, code,
   collective agreement, colloquium, comity, commandment, commission,
   committee, compact, conciliarism, conclave, concord, concordat,
   concourse, confab, confabulation, conference, conformity,
   confrontation, congregation, congress, consistory, consortium,
   constitutional convention, consuetude, consultation, contract,
   convenance, conventicle, conventional usage, conventionalism,
   conventionality, convocation, correctness, council, council fire,
   council of war, county convention, courtliness, covenant,
   covenant of salt, custom, dance, date, deal, decencies, decency,
   decorousness, decorum, dicker, dictum, diet, diocesan conference,
   diplomatic code, discussion, ecclesiastical council,
   ecumenical council, eisteddfod, elegance, elegancies,
   employment contract, entente, entente cordiale, established way,
   etiquette, exchange of views, exquisite manners,
   eyeball-to-eyeball encounter, fashion, festivity, fete, folkway,
   forgathering, form, formal agreement, formalities, formality,
   formula, forum, gathering, general principle, get-together,
   golden rule, good form, good manners, guideline, guiding principle,
   haute couture, high fashion, high-level talk, housewarming, huddle,
   imperative, interchange of views, international agreement,
   interview, ironclad agreement, law, league, legal agreement,
   legal contract, levee, manner, manners, maxim, meet, meeting,
   mitzvah, mode, moral, mores, mutual agreement,
   mutual-defense treaty, national convention, natural politeness,
   negotiations, news conference, nominating convention,
   nonaggression pact, norm, observance, ordinance, pact, paction,
   palaver, panel, parley, parochial church council,
   parochial council, party, plenary council, plenum,
   point of etiquette, politeness, politesse, political convention,
   pourparler, powwow, practice, praxis, precept,
   preliminary convention, presbytery, prescription, press conference,
   prevailing taste, principium, principle, prom, promise,
   proper thing, proprieties, propriety, protocol, punctilio,
   quadrennial circus, quiet good manners, quorum, rally, reception,
   regulation, rendezvous, ritual, rubric, rule, rules of conduct,
   seance, seemliness, seminar, session, settled principle, shindig,
   sit-in, sitting, social code, social conduct, social convention,
   social graces, social procedures, social usage, soiree, standard,
   standard behavior, standard usage, standing custom,
   state convention, stipulation, stream of fashion, style, summit,
   summit conference, summitry, swim, symposium, synod, tenet,
   time-honored practice, tradition, transaction, treaty, trend,
   turnout, understanding, union contract, usage, use, valid contract,
   vestry, vogue, wage contract, way, what is done, wont, wonting,
   working principle, working rule



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

CONVENTION, contracts, civil law. A general term which comprehends all kinds
of contracts, treaties, pacts, or agreements. It is defined to be the
consent of two or more persons to form with each other an engagement, or to
dissolve or change one which they had previously formed. Domat, Lois Civ. 1.
1, t. 1, s. 1 Dig. lib. 2, t. 14, 1. 1 Lib. 1, t. 1, 1. 1, 4 and 5; 1 Bouv.
Inst. n. 100.



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

CONVENTION, legislation. This term is applied to a selecting of the
delegates elected by the people for other purposes than usual legislation.
It is mostly used to denote all assembly to make or amend the constitution
of, a state, but it sometimes indicates an assembly of the delegates of the
people to nominate officers to be supported at an election.