Etymology Latin lucidus, from lūceō (“shine”) + -idus.
1
Clear; easily understood.
[T]he book, constructed in short, lucid episodes, can be satisfyingly read as a sequence of provocative talks, at once well informed and vatic.2014 September 26, Tom Payne, “Sapiens: a Brief History of Humankind by Yuval Noah Harari, review: 'urgent questions' [print version: The story of our species, 27 September 2014, p. R32]”, in The Daily Telegraph (Review):
2
Mentally rational; sane.
3
Bright, luminous, translucent, or transparent.
The atmosphere was unusually clear, as if loath to part with the daylight; but the moon, like a round of lucid snow, had risen on the sky; and a pale, soft gleam, came from the lamps amid the foliage.1837, L[etitia] E[lizabeth] L[andon], “The Fête”, in Ethel Churchill: Or, The Two Brides. […], volume III, London: Henry Colburn, […], →OCLC, page 57:
Pictures of growing spring and farms and homes, / With the Fourth-month eve at sundown, and the gray smoke lucid and bright, […]1865, Walt Whitman, “When Lilacs Last in the Dooryard Bloom’d”, in Sequel to Drum-Taps: When Lilacs Last in the Dooryard Bloom’d and other poems:
Etymology Latin lucidus, from lūceō (“shine”) + -idus.
1
A lucid dream.
The day before nightmare-initiated lucids, subjects reported more depressed feelings[…]1986, Benjamin B. Wolman, Montague Ullman, Handbook of states of consciousness, page 163: