/ Dictionary / Index A at. no.: Meaning and Definition of Find definitions for: at. no. Pronunciation: [key] atomic number. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Atna atoll Related Content Daily Word Quiz: penchant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language