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]