/ Dictionary / Index A atto-: Meaning and Definition of Find definitions for: atto- Pronunciation: [key] a combining form that enters into compounds denoting a unit one-quintillionth (10) the size of the unit named by the stem. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. attn. attorn Related Content Daily Word Quiz: fabricate Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language