From Latin 'lucidus' (bright, clear), from 'lux' (light), from PIE *lewk- — brightness as mental clarity.
Expressed clearly; easy to understand. Showing ability to think clearly, especially in intervals of confusion or insanity. Bright or luminous.
From Latin 'lucidus' (bright, clear, shining), derived from 'lucere' (to shine, to be light), from 'lux, lucis' (light). The Latin root traces to PIE *lewk- (light, brightness, to shine), one of the most productive roots in Indo-European, generating words for light and vision across the family. The semantic path is direct: that which glows is clear to see, hence intelligible and mentally transparent. English borrowed 'lucid' in the late 16th century in both its literal sense (giving off light) and its transferred sense (clear to the mind, intellectually transparent). The doublet 'lucent' arrived slightly earlier. The same PIE root gives English 'luna' (moon, the shining one), 'illustrate' (to throw light on), and 'luster.' Key roots: lūx (Latin: "light"), *lewk- (Proto-Indo-European: "light, brightness").
The name 'Lucifer' — meaning 'light-bearer,' from Latin 'lūx' (light) + 'ferre' (to carry) — was originally the Latin name for the morning star, the planet Venus when it appears before dawn. It was not a name for the devil until early Christian writers applied Isaiah 14:12 ('How art thou fallen from heaven, O Lucifer, son of the morning') to Satan. The brightest name in Latin astronomy became the darkest name in Christian theology.