The Collaborative International Dictionary of English v.0.48:

Concentrate \Con*cen"trate\ (? or ?), v. t. [imp. & p. p.
   Concentrated; p. pr. & vb. n. Concentrating.] [Pref. con-
   + L. centrum center. Cf. Concenter.]
   1. To bring to, or direct toward, a common center; to unite
      more closely; to gather into one body, mass, or force; to
      fix; as, to concentrate rays of light into a focus; to
      concentrate the attention.
      [1913 Webster]

            (He) concentrated whole force at his own camp.
                                                  --Motley.
      [1913 Webster]

   2. To increase the strength and diminish the bulk of, as of a
      liquid or an ore; to intensify, by getting rid of useless
      material; to condense; as, to concentrate acid by
      evaporation; to concentrate by washing; -- opposed to
      dilute.
      [1913 Webster]

            Spirit of vinegar concentrated and reduced to its
            greatest strength.                    --Arbuthnot.

   Syn: To combine; to condense; to consolidate.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Concentrate \Con*cen"trate\ (? or ?), v. i.
   To approach or meet in a common center; to consolidate; as,
   population tends to concentrate in cities.
   [1913 Webster]

WordNet (r) 3.0 (2006):

concentrate
    n 1: the desired mineral that is left after impurities have been
         removed from mined ore [syn: dressed ore, concentrate]
    2: a concentrated form of a foodstuff; the bulk is reduced by
       removing water
    3: a concentrated example of something; "the concentrate of
       contemporary despair"
    v 1: make denser, stronger, or purer; "concentrate juice"
    2: direct one's attention on something; "Please focus on your
       studies and not on your hobbies" [syn: concentrate,
       focus, center, centre, pore, rivet]
    3: make central; "The Russian government centralized the
       distribution of food" [syn: centralize, centralise,
       concentrate] [ant: decentralise, decentralize,
       deconcentrate]
    4: make more concise; "condense the contents of a book into a
       summary" [syn: digest, condense, concentrate]
    5: draw together or meet in one common center; "These groups
       concentrate in the inner cities"
    6: compress or concentrate; "Congress condensed the three-year
       plan into a six-month plan" [syn: condense, concentrate,
       contract]
    7: be cooked until very little liquid is left; "The sauce should
       reduce to one cup" [syn: boil down, reduce, decoct,
       concentrate]
    8: cook until very little liquid is left; "The cook reduced the
       sauce by boiling it for a long time" [syn: reduce, boil
       down, concentrate]

Moby Thesaurus II by Grady Ward, 1.0:

168 Moby Thesaurus words for "concentrate":
   abbreviate, accelerate, aggravate, approach, assemble,
   attend closely to, attend to business, be alert, be obsessed with,
   beef up, blow up, bring into focus, bring together, center,
   centralize, circumscribe, close, close in, close up, close with,
   cluster, coarct, collect, come together, compact, complicate,
   compress, concenter, concentralize, concentrate on,
   concentrate the mind, concentrate upon, concentrated,
   concentration, condense, congest, congregate, consolidate,
   constant, constrict, constringe, contract, converge, cram, cramp,
   crowd, curtail, decoct, decoction, decrease, deepen, densen,
   densify, direct, distill, distillate, distillation, double, draw,
   draw in, draw together, elixir, enhance, essence, essentialize,
   establish, exacerbate, exaggerate, exclusive, express, extract,
   extraction, fall in with, firm, fix, fix on, fixate, fixed,
   focalize, focus, focus on, funnel, gather, give heed, group, heap,
   heat up, heighten, hop up, hot up, infuse, infusion, integrate,
   intensify, intersect, jam, jazz up, key up, knit, look lively,
   look out, look sharp, magnify, make complex, mass, meet, melt down,
   miss nothing, narrow, narrow the gap, nip, pay attention, pay heed,
   pile, pinch, press, press out, pucker, pucker up, purification,
   purse, put, quintessence, ram down, ramify, redouble, reduce,
   refine, refinement, reinforce, render, rivet, run together,
   scrutinize, set, settle, sharpen, shorten, shrink, soak, solidify,
   soup up, spirit, squeeze, steadfast, steady, steep, step up,
   strangle, strangulate, strengthen, study, take care, take heed,
   taper, think, triple, undistracted, undivided, unfaltering, unite,
   unremitting, unwavering, watch out, whet, wring, wring out,
   wrinkle