The Collaborative International Dictionary of English v.0.48:

Absorption \Ab*sorp"tion\, n. [L. absorptio, fr. absorbere. See
   Absorb.]
   1. The act or process of absorbing or sucking in anything, or
      of being absorbed and made to disappear; as, the
      absorption of bodies in a whirlpool, the absorption of a
      smaller tribe into a larger.
      [1913 Webster]

   2. (Chem. & Physics) An imbibing or reception by molecular or
      chemical action; as, the absorption of light, heat,
      electricity, etc.
      [1913 Webster]

   3. (Physiol.) In living organisms, the process by which the
      materials of growth and nutrition are absorbed and
      conveyed to the tissues and organs.
      [1913 Webster]

   4. Entire engrossment or occupation of the mind; as,
      absorption in some employment.
      [1913 Webster]

WordNet (r) 3.0 (2006):

absorption
    n 1: (chemistry) a process in which one substance permeates
         another; a fluid permeates or is dissolved by a liquid or
         solid [syn: absorption, soaking up]
    2: (physics) the process in which incident radiated energy is
       retained without reflection or transmission on passing
       through a medium; "the absorption of photons by atoms or
       molecules"
    3: the social process of absorbing one cultural group into
       harmony with another [syn: assimilation, absorption]
    4: the process of absorbing nutrients into the body after
       digestion [syn: assimilation, absorption]
    5: complete attention; intense mental effort [syn:
       concentration, engrossment, absorption, immersion]
    6: the mental state of being preoccupied by something [syn:
       preoccupation, preoccupancy, absorption, engrossment]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "absorption":
   Walter Mitty, ablation, absence of mind, absentmindedness,
   absorbed attention, absorbency, absorbent, abstractedness,
   abstraction, adsorbent, adsorption, application, assimilation,
   attrition, bemusement, bile, blotter, blotting, blotting paper,
   brown study, burning up, castle-building, chemisorption,
   chemosorption, close study, concentration, consumption,
   contemplation, contemplativeness, daydream, daydreamer,
   daydreaming, deep study, deep thought, depletion, depth of thought,
   digestion, digestive system, drain, dream, dreaming, eating up,
   embarrassment, endosmosis, engagement, engrossment, enmeshment,
   entanglement, erosion, exhaustion, exosmosis, expending,
   expenditure, fantasy, fantasying, finishing, fit of abstraction,
   gastric juice, gastrointestinal tract, imbibing, immersion,
   implication, impoverishment, inclusion, infiltration, ingestion,
   intentness, intestinal juice, involution, involvement, liver,
   meditation, melancholy, monomania, mooning, moonraking, muse,
   musefulness, musing, muted ecstasy, obsession, osmosis, pancreas,
   pancreatic digestion, pancreatic juice, pensiveness, percolation,
   pipe dream, pipe-dreaming, predigestion, preoccupation,
   profound thought, rapt attention, reflectiveness, relation,
   reverie, saliva, salivary digestion, salivary glands,
   secondary digestion, seepage, single-mindedness, soaking-up,
   sorption, speculativeness, spending, sponge, sponging, squandering,
   stargazing, studiousness, study, submersion, taking-in,
   thoughtfulness, trance, using up, wastage, waste, wastefulness,
   wasting away, wearing away, wearing down, wistfulness,
   woolgathering