Definition of atto- in English:

atto-

  • (used in units of measurement) denoting a factor of 10⁻¹⁸

    • ‘attowatt’

Pronunciation

atto-

/ˈaˌtō/ /ˈæˌtoʊ/ /ˈadō/ /ˈædoʊ/

Origin

From Danish or Norwegian atten ‘eighteen’.