Crossword clues for the word: ATTO The word ATTO has appeared as an answer to at least 7 crossword clues on different crossword puzzles. Crossword clues for ATTO Showing 1 to 7 of 7 results Clue(s) It means ''billionth of a billionth'' Metric prefix that's the reciprocal of exa- One degree smaller than femto- One-quintillionth: Prefix Prefix for 'quintillionth' Quintillionth, in metric prefixes Quintillionth: Prefix