Definition of atto- in English:

atto-

Pronunciation: /ˈatəʊ/

combining form

(Used in units of measurement) denoting a factor of 10−18: attowatt

Origin

from Danish or Norwegian atten 'eighteen'.

Definition of atto- in:

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Word of the day neoteny
Pronunciation: niːˈɒt(ə)ni
noun
retention of juvenile features in the adult animal