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1. (of language) transparently clear; easily understandable
   "lucid directions";
   - limpid, 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";
   - coherent, logical
    
4. Transmitting light; able to be seen through with clarity
   "lucid air";
   - crystalline, crystal clear, limpid, pellucid, transparent

See also: clear, rational, sane

Encyclopedia: Lucid