The Collaborative International Dictionary of English v.0.48:

Multiply \Mul"ti*ply\, v. t. [imp. & p. p. Multiplied; p. pr.
   & vb. n. Multiplying.] [F. multiplier, L. multiplicare, fr.
   multiplex manifold. See Multitude, Complex.]
   1. To increase in number; to make more numerous; to add
      quantity to.
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            Impunity will multiply motives to disobedience.
                                                  --Ames.
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   2. (Math.) To add (any given number or quantity) to itself a
      certain number of times; to find the product of by
      multiplication; thus 7 multiplied by 8 produces the number
      56; to multiply two numbers. See the Note under
      Multiplication.
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   3. To increase (the amount of gold or silver) by the arts of
      alchemy. [Obs.]
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   Multiplying gear (Mach.), gear for increasing speed.

   Multiplying lens. (Opt.) See under Lens.
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The Collaborative International Dictionary of English v.0.48:

Multiply \Mul"ti*ply\, v. i.
   1. To become greater in number; to become numerous.
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            When men began to multiply on the face of the earth,
            and daughters were born unto them.    --Gen. vi. 1.
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   2. To increase in extent and influence; to spread.
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            The word of God grew and multiplied.  --Acts xii.
                                                  24.
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   3. To increase amount of gold or silver by the arts of
      alchemy. [Obs.] --Chaucer.
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WordNet (r) 3.0 (2006):

multiply
    adv 1: in several ways; in a multiple manner; "they were
           multiply checked for errors" [ant: singly]
    v 1: combine by multiplication; "multiply 10 by 15" [ant:
         divide, fraction]
    2: combine or increase by multiplication; "He managed to
       multiply his profits" [syn: multiply, manifold]
    3: have young (animals) or reproduce (organisms); "pandas rarely
       breed in captivity"; "These bacteria reproduce" [syn:
       breed, multiply]
    4: have offspring or produce more individuals of a given animal
       or plant; "The Bible tells people to procreate" [syn:
       reproduce, procreate, multiply]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "multiply":
   abound with, accrue, accumulate, add, advance, aggrandize,
   algebraize, appreciate, army, augment, balloon, be alive with,
   be fruitful, be productive, bear, beef up, beget, bloat, boom,
   boost, breed, breed true, bristle with, broaden, build, burst with,
   calculate, cast, cipher, cloud, clutter, compute, copulate,
   crawl with, creep with, crescendo, crossbreed, crowd, crush,
   develop, divide, dope out, drove, engender, enlarge, estimate,
   expand, extend, extract roots, father, figure, figure in,
   figure out, flock, fructify, gain, gain strength, generate, get,
   get ahead, go up, grow, heighten, horde, host, inbreed, increase,
   intensify, jam, legion, magnify, make love, measure, mother, mount,
   multitude, numbers, oodles, outbreed, overflow with, pack, press,
   procreate, produce, proliferate, propagate, pullulate,
   pullulate with, push, quantities, reckon, reproduce,
   reproduce in kind, rise, rout, run up, score, scores, shoot up,
   sire, snowball, spread, squash, strengthen, subtract, swarm with,
   swell, take account of, tally, teem, teem with, throng,
   throng with, upsurge, wax, widen, work out