The Collaborative International Dictionary of English v.0.48:

Lucid \Lu"cid\, a. [L. lucidus, fr. lux, lucis, light. See
   Light, n.]
   1. Shining; bright; resplendent; as, the lucid orbs of
      heaven.
      [1913 Webster]

            Lucid, like a glowworm.               --Sir I.
                                                  Newton.
      [1913 Webster]

            A court compact of lucid marbles.     --Tennyson.
      [1913 Webster]

   2. Clear; transparent. " Lucid streams." --Milton.
      [1913 Webster]

   3. Presenting a clear view; easily understood; clear.
      [1913 Webster]

            A lucid and interesting abstract of the debate.
                                                  --Macaulay.
      [1913 Webster]

   4. Bright with the radiance of intellect; not darkened or
      confused by delirium or madness; marked by the regular
      operations of reason; as, a lucid interval.
      [1913 Webster]

   Syn: Luminous; bright; clear; transparent; sane; reasonable.
        See Luminous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

lucid
    adj 1: (of language) transparently clear; easily understandable;
           "writes in a limpid style"; "lucid directions"; "a
           luculent oration"- Robert Burton; "pellucid prose"; "a
           crystal clear explanation"; "a perspicuous argument"
           [syn: limpid, lucid, luculent, pellucid, crystal
           clear, perspicuous]
    2: having a clear mind; "a lucid moment in his madness"
    3: 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]
    4: transmitting light; able to be seen through with clarity;
       "the cold crystalline water of melted snow"; "crystal clear
       skies"; "could see the sand on the bottom of the limpid
       pool"; "lucid air"; "a pellucid brook"; "transparent crystal"
       [syn: crystalline, crystal clear, limpid, lucid,
       pellucid, transparent]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "lucid":
   Attic, Ciceronian, all there, apprehensible, balanced, beaming,
   bright, brilliant, chaste, classic, clean-cut, clear,
   clear as crystal, clear as day, clear-cut, clearheaded,
   clearminded, cloudless, coherent, compos mentis, comprehensible,
   connected, consistent, crisp, crystal, crystal-clear, crystalline,
   defined, definite, diaphane, diaphanous, direct, distinct, easy,
   effulgent, elegant, explicit, express, fathomable, filmy, finished,
   gauzy, gossamer, gossamery, graceful, gracile, graspable,
   healthy-minded, incandescent, intelligible, knowable, lambent,
   light, light-pervious, lightish, lightsome, limpid, loud and clear,
   lucent, luculent, luminous, lustrous, mentally sound, natural,
   neat, nonopaque, normal, of sound mind, peekaboo, pellucid,
   perspicuous, plain, polished, pure, radiant, rational, reasonable,
   refined, refulgent, relucent, restrained, revealing, right, round,
   sane, sane-minded, see-through, semipellucid, semitranslucent,
   sensible, serene, sheer, simple, sound, sound-minded,
   straightforward, tasteful, terse, thin, together, translucent,
   translucid, transparent, transpicuous, trim, unaffected,
   unambiguous, unclouded, unconfused, unequivocal, univocal,
   unlabored, unmistakable, unobscured, well-defined, wholesome



The Free On-line Dictionary of Computing (30 December 2018):

LUCID

   1. Early query language, ca. 1965, System Development Corp,
   Santa Monica, CA.  [Sammet 1969, p.701].

   2. A family of dataflow languages descended from ISWIM,
   lazy but first-order.

   Ashcroft & Wadge , 1981.

   They use a dynamic demand driven model.  Statements are
   regarded as equations defining a network of processors and
   communication lines, through which the data flows.  Every data
   object is thought of as an infinite stream of simple values,
   every function as a filter.  Lucid has no data
   constructors such as arrays or records.  Iteration is
   simulated with 'is current' and 'fby' (concatenation of
   sequences).  Higher-order functions are implemented using pure
   dataflow and no closures or heaps.

   ["Lucid: The Dataflow Language" by Bill Wadge
    and Ed Ashcroft, c. 1985].  ["Lucid, the
   Dataflow Programming Language", W. Wadge, Academic Press
   1985].

   (1995-02-16)