combining form Etymology: International Scientific Vocabulary, from Danish or Norwegian femten fifteen, from Old Norse fimmtān; akin to Old English fīftēne fifteen one quadrillionth (10 -15 ) part of
FEMTO-
Meaning of FEMTO- in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012