one quadrillionth part of; the factor 10: femtosecond
Origin of femto-; from Danish femten, fifteen ; from Old Danish femtan ; from fem, five + -tjan, akin to Old English -tyne, -teen
One quadrillionth (10−15): femtovolt.
Origin of femto-From Danish and Norwegian femten, fifteen, from Old Norse fimmtan; see penkwe in Indo-European roots.
From Danish femten (“fifteen”).