/ Dictionary / Index P phosphorous acid: Meaning and Definition of Find definitions for: phos'phorous ac'id Pronunciation: [key] — Chem. Chem. a colorless, crystalline, water-soluble acid of phosphorus, HPO, from which phosphites are derived. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. phosphorous phosphorus Related Content Daily Word Quiz: cygnet Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language