The Collaborative International Dictionary of English v.0.48:

Business \Busi"ness\ (b[i^]z"n[e^]s), n.; pl. Businesses
   (b[i^]z"n[e^]s*[e^]z). [From Busy.]
   1. That which busies one, or that which engages the time,
      attention, or labor of any one, as his principal concern
      or interest, whether for a longer or shorter time;
      constant employment; regular occupation; as, the business
      of life; business before pleasure.
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            Wist ye not that I must be about my Father's
            business?                             --Luke ii. 49.
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   2. Any particular occupation or employment engaged in for
      livelihood or gain, as agriculture, trade, art, or a
      profession. "The business of instruction." --Prescott.
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   3. Financial dealings; buying and selling; traffic in
      general; mercantile transactions.
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            It seldom happens that men of a studious turn
            acquire any degree of reputation for their knowledge
            of business.                          --Bp. Popteus.
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   4. That which one has to do or should do; special service,
      duty, or mission.
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            The daughter of the King of France,
            On serious business, craving quick despatch,
            Importunes personal conference.       --Shak.
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            What business has the tortoise among the clouds?
                                                  --L'Estrange.
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   5. Affair; concern; matter; -- used in an indefinite sense,
      and modified by the connected words.
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            It was a gentle business, and becoming
            The action of good women.             --Shak.
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            Bestow
            Your needful counsel to our business. --Shak.
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   6. (Drama) The position, distribution, and order of persons
      and properties on the stage of a theater, as determined by
      the stage manager in rehearsal.
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   7. Care; anxiety; diligence. [Obs.] --Chaucer.
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   To do one's business, to ruin one. [Colloq.] --Wycherley.

   To make (a thing) one's business, to occupy one's self with
      a thing as a special charge or duty. [Colloq.]

   To mean business, to be earnest. [Colloq.]
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   Syn: Affairs; concern; transaction; matter; engagement;
        employment; calling; occupation; trade; profession;
        vocation; office; duty.
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WordNet (r) 3.0 (2006):

business
    n 1: a commercial or industrial enterprise and the people who
         constitute it; "he bought his brother's business"; "a small
         mom-and-pop business"; "a racially integrated business
         concern" [syn: business, concern, business concern,
         business organization, business organisation]
    2: the activity of providing goods and services involving
       financial and commercial and industrial aspects; "computers
       are now widely used in business" [syn: commercial
       enterprise, business enterprise, business]
    3: the principal activity in your life that you do to earn
       money; "he's not in my line of business" [syn: occupation,
       business, job, line of work, line]
    4: a rightful concern or responsibility; "it's none of your
       business"; "mind your own business"
    5: an immediate objective; "gossip was the main business of the
       evening"
    6: the volume of commercial activity; "business is good today";
       "show me where the business was today"
    7: business concerns collectively; "Government and business
       could not agree" [syn: business, business sector]
    8: customers collectively; "they have an upper class clientele"
       [syn: clientele, patronage, business]
    9: incidental activity performed by an actor for dramatic
       effect; "his business with the cane was hilarious" [syn:
       business, stage business, byplay]

Moby Thesaurus II by Grady Ward, 1.0:

214 Moby Thesaurus words for "business":
   Aktiengesellschaft, acting, action, activeness, activism, activity,
   affair, aktiebolag, allegiance, area, art, assigned task, attempt,
   balance of trade, big business, body corporate, bounden duty,
   buffoonery, burden, business dealings, business establishment,
   call of duty, calling, career, career building, careerism, cartel,
   chamber of commerce, characterization, charge, combine, commerce,
   commercial, commercial affairs, commercial enterprise,
   commercial relations, commitment, compagnie, company, concern,
   concernment, conglomerate, conglomerate corporation,
   consolidating company, consortium, contract, copartnership,
   corporate body, corporation, craft, custom, deal, dealing,
   dealings, dedication, deference, devoir, devotion, dingus,
   diversified corporation, dofunny, doing, doings,
   duties and responsibilities, duty, effort, employment, engagement,
   enterprise, establishment, ethics, fair trade, fealty, firm,
   free trade, function, gadget, gag, game, gizmo, goings-on, ham,
   hammy acting, handicraft, hoke, hokum, holding company, homage,
   hootenanny, house, imperative, impersonation, industrial, industry,
   intercourse, interest, issue, jigger, job, joint-stock association,
   joint-stock company, lifework, line, line of business,
   line of duty, line of work, lookout, loyalty, market, marketing,
   matter, mercantile, mercantile business, merchant, merchantry,
   metier, militancy, mimesis, mimicking, mimicry, miming, mission,
   motion, movement, multilateral trade, mummery, must, mystery,
   number, obligation, occasions, occupation, office, onus,
   operating company, operation, organization, ought, outfit,
   overacting, palaver, pantomiming, partnership, patter, performance,
   performing, personation, place, plan, playacting, playing,
   plunderbund, point, political activism, pool, portrayal, practice,
   problem, proceeding, proceedings, profession, program, project,
   projection, proposition, proprietorship, province, public utility,
   pursuit, question, racket, reciprocal trade, representation,
   respect, responsibility, restraint of trade, retail, role,
   self-imposed duty, slapstick, small business, specialization,
   specialty, stage business, stage directions, stage presence, stir,
   stock company, stunt, subject, syndicate, taking a role, task,
   the business world, the marketplace, thing, thingumajig, topic,
   trade, trade association, trading, traffic, transaction, truck,
   trust, undertaking, unilateral trade, utility, venture, vocation,
   walk, walk of life, wholesale, work