/ Dictionary / Index U usnic acid: Meaning and Definition of Find definitions for: us'nic ac'id Pronunciation: (us'nik), [key] — Pharm. Pharm. an antibacterial substance, CHO, derived from lichens of the genus Usnea. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. USNG USNR Related Content Daily Word Quiz: spectral Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language