(1):

(n.) 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.

(2):

(n.) Presenting a clear view; easily understood; clear.

(3):

(n.) Clear; transparent.

(4):

(n.) Shining; bright; resplendent; as, the lucid orbs of heaven.