pellucid

The adjective is a learned borrowing from Latin pellūcidus, perlucidus (“transparent, pellucid; very bright; very understandable”), from per- (prefix meaning ‘through; throughout; completely, thoroughly’) + lūcidus (“clear; full of light, bright, shining; (figuratively) easily understood, clear, lucid”) (from lūceō (“to shine; to become visible, show through; (figuratively) to be apparent, conspicuous, or evident”) (ultimately from Proto-Indo-European *lewk- (“bright; to see; to shine”)) + -idus (suffix meaning ‘tending to’ forming adjectives)).

The noun is derived from the adjective.