The Collaborative International Dictionary of English v.0.48:

Coherent \Co*her"ent\, a. [L. cohaerens, p. pr. See Cohere.]
   1. Sticking together; cleaving; as the parts of bodies; solid
      or fluid. --Arbuthnot.
      [1913 Webster]

   2. Composed of mutually dependent parts; making a logical
      whole; consistent; as, a coherent plan, argument, or
      discourse.
      [1913 Webster]

   3. Logically consistent; -- applied to persons; as, a
      coherent thinker. --Watts.
      [1913 Webster]

   4. Suitable or suited; adapted; accordant. [Obs.]
      [1913 Webster]

            Instruct my daughter how she shall persever,
            That time and place, with this deceit so lawful,
            May prove coherent.                   --Shak.
      [1913 Webster]

   5. (Physics, Optics) Of or pertaining to electromagnetic
      waves that have a constant phase relationship with each
      other; having identical phase at all points; as, the
      coherent light produced by a laser.
      [PJC]

WordNet (r) 3.0 (2006):

coherent
    adj 1: marked by an orderly, logical, and aesthetically
           consistent relation of parts; "a coherent argument" [syn:
           coherent, consistent, logical, ordered] [ant:
           incoherent]
    2: capable of thinking and expressing yourself in a clear and
       consistent manner; "a lucid thinker"; "she was more coherent
       than she had been just after the accident" [syn: coherent,
       logical, lucid]
    3: (physics) of waves having a constant phase relation [ant:
       incoherent]
    4: sticking together; "two coherent sheets"; "tenacious burrs"
       [syn: coherent, tenacious]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "coherent":
   accordant, adhering, affirmative, agglutinate, agreeable, agreeing,
   answerable, articulate, at one, clean-cut, clear, clear as crystal,
   clear as day, clear-cut, cleaving, clinging, coexistent,
   coexisting, cohering, cohesive, coincident, coinciding,
   commensurate, compatible, comprehensible, concordant, concurring,
   conformable, congenial, congruent, congruous, connected,
   consentaneous, consentient, consistent, consonant, cooperating,
   cooperative, correspondent, corresponding, crisp, crystal-clear,
   crystalline, defined, definite, direct, distinct, en rapport,
   equivalent, explicit, express, harmonious, holding together,
   impartible, in accord, in agreement, in rapport, in sync,
   in synchronization, inaccordance, indiscerptible, indissoluble,
   indissolvable, indivisible, infrangible, infusible, inharmony,
   inseparable, insoluble, intelligible, like-minded, limpid, logical,
   loud and clear, lucid, luminous, of a piece, of like mind,
   of one mind, on all fours, orderly, organized, pellucid,
   perspicuous, plain, positive, proportionate, rational, reasonable,
   reconcilable, self-consistent, sensible, simple, sticking,
   straightforward, stuck, symbiotic, synchronized, synchronous,
   translucent, transparent, transpicuous, unambiguous, unanimous,
   unconfused, understandable, undividable, unequivocal, unified,
   uniform, unisonant, unisonous, univocal, unmistakable,
   well-defined, well-ordered