Definition of femto- combining form from the Oxford Advanced Learner's Dictionary

 

femto-

 combining form
combining form
BrE BrE//ˈfemtəʊ//
 
; NAmE NAmE//ˈfemtoʊ//
 
(specialist)
 
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(in units of measurement) 10-15 a femtosecond Word Originfrom Danish or Norwegian femten ‘fifteen’.
See the Oxford Advanced American Dictionary entry: femto-