The Collaborative International Dictionary of English v.0.48:

Maintain \Main*tain\, v. t. [imp. & p. p. Maintained; p. pr. &
   vb. n. Maintaining.] [OE. maintenen, F. maintenir,
   properly, to hold by the hand; main hand (L. manus) + F.
   tenir to hold (L. tenere). See Manual, and Tenable.]
   1. To hold or keep in any particular state or condition; to
      support; to sustain; to uphold; to keep up; not to suffer
      to fail or decline; as, to maintain a certain degree of
      heat in a furnace; to maintain a fence or a railroad; to
      maintain the digestive process or powers of the stomach;
      to maintain the fertility of soil; to maintain present
      reputation.
      [1913 Webster]

   2. To keep possession of; to hold and defend; not to
      surrender or relinquish.
      [1913 Webster]

            God values . . . every one as he maintains his post.
                                                  --Grew.
      [1913 Webster]

   3. To continue; not to suffer to cease or fail.
      [1913 Webster]

            Maintain talk with the duke.          --Shak.
      [1913 Webster]

   4. To bear the expense of; to support; to keep up; to supply
      with what is needed.
      [1913 Webster]

            Glad, by his labor, to maintain his life.
                                                  --Stirling.
      [1913 Webster]

            What maintains one vice would bring up two children.
                                                  --Franklin.
      [1913 Webster]

   5. To affirm; to support or defend by argument.
      [1913 Webster]

            It is hard to maintain the truth, but much harder to
            be maintained by it.                  --South.
      [1913 Webster]

   Syn: To assert; vindicate; allege. See Assert.
        [1913 Webster]

WordNet (r) 3.0 (2006):

maintain
    v 1: keep in a certain state, position, or activity; e.g., "keep
         clean"; "hold in place"; "She always held herself as a
         lady"; "The students keep me on my toes" [syn: keep,
         maintain, hold]
    2: keep in safety and protect from harm, decay, loss, or
       destruction; "We preserve these archeological findings"; "The
       old lady could not keep up the building"; "children must be
       taught to conserve our national heritage"; "The museum
       curator conserved the ancient manuscripts" [syn: conserve,
       preserve, maintain, keep up]
    3: supply with necessities and support; "She alone sustained her
       family"; "The money will sustain our good cause"; "There's
       little to earn and many to keep" [syn: sustain, keep,
       maintain]
    4: state categorically [syn: assert, asseverate, maintain]
    5: have and exercise; "wield power and authority" [syn: wield,
       exert, maintain]
    6: maintain for use and service; "I keep a car in the
       countryside"; "She keeps an apartment in Paris for her
       shopping trips" [syn: keep, maintain]
    7: maintain by writing regular records; "keep a diary";
       "maintain a record"; "keep notes" [syn: keep, maintain]
    8: state or assert; "He maintained his innocence" [syn:
       maintain, defend]
    9: support against an opponent; "The appellate court upheld the
       verdict" [syn: uphold, maintain]
    10: stick to correctly or closely; "The pianist kept time with
        the metronome"; "keep count"; "I cannot keep track of all my
        employees" [syn: observe, keep, maintain]

Moby Thesaurus II by Grady Ward, 1.0:

270 Moby Thesaurus words for "maintain":
   abide, accommodate, account as, advocate, affirm, afford,
   afford support, allege, allege in support, announce, annunciate,
   answer, argue, argue for, assert, assever, asseverate, assume,
   aver, avouch, avow, back, back up, be afraid, bear, bear out,
   bear up, bide, blow to, bolster, bolster up, bottle up, brace,
   brook no denial, buoy up, buttress, care for, carry, carry on,
   champion, claim, clothe, conceive, conserve, consider, contend,
   contend for, continue, continue to be, contribute, copyright,
   cork up, correct, counter, cradle, crutch, cultivate, cushion,
   daresay, declare, deem, defeat time, defend, defy time, donate,
   dwell, emphasize, endow, endure, enunciate, espouse, esteem,
   estimate, exist, expect, express, extend, fancy, fight for, fill,
   fill up, finance, find, fund, furnish, give, give support, go on,
   go treat, guard, guess, have, have a hunch, have an idea,
   have an impression, have an inkling, have the idea, hold, hold as,
   hold in, hold on, hold out, hold up, husband, imagine, inhibit,
   insist, insist on, insist upon, invest, issue a manifesto, judge,
   justify, keep, keep afloat, keep alive, keep going, keep in,
   keep intact, keep inviolate, keep on, keep safe, keep up, last,
   last long, last out, lay down, lend support, lengthen, live,
   live on, live through, lock in, look after, look upon as, mainstay,
   make a plea, make available, make provision for, manage, manifesto,
   not destroy, not endanger, not expend, not use up, not waste,
   nurture, opine, patent, pay the bill, perdure, perennate,
   perpetuate, persist, persist in, pillow, plead for, predicate,
   prepare, present, preserve, press, presume, prevail, proclaim,
   profess, prolong, pronounce, prop, prop up, protect, protest,
   protract, provide, provide for, put, put it, rebut, reckon,
   recruit, rectify, refute, regard, register, reinforce, remain,
   replenish, reply, repress, respond, retain, right, riposte, run,
   run on, save, save up, say, say in defense, set down, set down as,
   set up, shore, shore up, shoulder, spare, speak, speak for,
   speak out, speak up, speak up for, stand, stand by, stand drinks,
   stand for, stand on, stand to, stand treat, stand up for, state,
   stay, stay on, stick to, stick up for, stock, store, stress,
   submit, subsidize, subsist, subvention, subventionize, supply,
   support, suppose, suppress, surmise, survive, suspect, sustain,
   take, take care of, take for, take it, take no denial, tarry,
   testify, think, tide over, treat, treat to, trow, underbrace,
   undergird, underlie, underpin, underset, upbear, uphold, upkeep,
   urge, urge reasons for, view as, vindicate, vouchsafe, warrant,
   wear, wear well, ween, yield