Definitions of lucid:

• adjective:   having a clear mind

  Example: "A lucid moment in his madness"

• adjective:   (of language) transparently clear; easily understandable

  Example: "Lucid directions"

• adjective:   capable of thinking and expressing yourself in a clear and consistent manner

  Example: "A lucid thinker"

• adjective:   transmitting light; able to be seen through with clarity

  Example: "Lucid air"