The Collaborative International Dictionary of English v.0.48:

Concentration \Con`cen*tra"tion\, n. [Cf. F. concentration.]
   1. The act or process of concentrating; the process of
      becoming concentrated, or the state of being concentrated;
      concentration.
      [1913 Webster]

            Concentration of the lunar beams.     --Boyle.
      [1913 Webster]

            Intense concetration of thought.      --Sir J.
                                                  Herschel.
      [1913 Webster]

   2. The act or process of reducing the volume of a liquid, as
      by evaporation.
      [1913 Webster]

            The acid acquires a higher degree of concentration.
                                                  --Knight.
      [1913 Webster]

   3. (Metal.) The act or process of removing the dress of ore
      and of reducing the valuable part to smaller compass, as
      by currents of air or water.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dephlegmation \De`phleg*ma"tion\, n. [Cf. F. d['e]flegmation.]
   (Chem.)
   The operation of separating water from spirits and acids, by
   evaporation or repeated distillation; -- called also
   concentration, especially when acids are the subject of it.
   [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

concentration
    n 1: the strength of a solution; number of molecules of a
         substance in a given volume
    2: the spatial property of being crowded together [syn:
       concentration, density, denseness, tightness,
       compactness] [ant: dispersion, distribution]
    3: strengthening the concentration (as of a solute in a mixture)
       by removing diluting material [ant: dilution]
    4: increase in density
    5: complete attention; intense mental effort [syn:
       concentration, engrossment, absorption, immersion]
    6: bringing together military forces
    7: great and constant diligence and attention [syn: assiduity,
       assiduousness, concentration]

Moby Thesaurus II by Grady Ward, 1.0:

241 Moby Thesaurus words for "concentration":
   abbreviation, absorbed attention, absorption, abstraction,
   accelerando, acceleration, advertence, advertency, agglutination,
   aggravation, alertness, application, approach, ardor, assiduity,
   assiduousness, astriction, astringency, asymptote, attention,
   attention span, attentiveness, awareness, beefing-up, blowing up,
   blowup, bottleneck, brown study, bulldog tenacity, care, centering,
   centralism, centralization, cervix, circumscription, close study,
   clumping, clustering, coarctation, collision course, compactedness,
   compaction, compression, compressure, concentralization,
   concentrate, concourse, concretion, concurrence, condensation,
   confluence, conflux, congress, consciousness, consideration,
   consolidation, constancy, constriction, constringency,
   contemplation, contemplativeness, contraction, contracture,
   convergence, converging, crossing, curtailment, debate, decoction,
   decrease, deep study, deep thought, deepening, deliberation,
   densification, diligence, diminuendo, distillate, distillation,
   dogged perseverance, doggedness, ear, earnestness, elixir,
   endurance, energeticalness, energy, engagement, engrossment,
   enhancement, enthrallment, essence, exacerbation, exaggeration,
   explosion, expression, extract, extraction, fervor, fidelity,
   firmness, firmness of mind, fixedness, flintiness, focalization,
   focus, focusing, funnel, hardening, heating-up, heed, heedfulness,
   heightening, hourglass, hourglass figure, hub, immersion,
   indefatigability, industriousness, industry, inflexibility,
   information explosion, infusion, insistence, insistency,
   intensification, intentiveness, intentness, involvement, isthmus,
   knitting, laboriousness, loyalty, magnification, meditation,
   meeting, melancholy, mindfulness, monomania, musing,
   mutual approach, narrow place, narrowing, narrowing gap, neck,
   note, notice, observance, observation, obsession, obstinacy,
   patience, patience of Job, pensiveness, permanence, perseverance,
   persistence, persistency, pertinaciousness, pertinacity, pickup,
   plodding, plugging, pooling, population explosion, preoccupation,
   pressing, profound thought, puckering, purification, pursing,
   quintessence, radius, rapt attention, redoubling, reduction,
   refinement, reflectiveness, regard, regardfulness, reinforcement,
   relentlessness, remark, rendering, rendition, resolution, respect,
   reverie, rigidity, sedulity, sedulousness, settledness, shortening,
   single-mindedness, singleness of purpose, slogging, soaking,
   solidification, speculativeness, speedup, spirit, spokes,
   squeezing, stability, stamina, staunchness, staying power,
   steadfastness, steadiness, steeliness, steeping, step-up,
   stick-to-itiveness, stranglement, strangulation, strengthening,
   strenuousness, striction, stricture, stubbornness, studiousness,
   study, submersion, systole, tangent, tenaciousness, tenacity,
   thought, thoughtfulness, tightening, tirelessness, trueness,
   unremittingness, unsparingness, unswerving attention,
   unyieldingness, vehemence, wasp waist, wistfulness, wrinkling,
   zealousness