# femto-

> English word · Prefix

## Definitions
1. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁵ (short scale quadrillionth or long scale billiardth).

## Etymology
From Danish femten (“fifteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains atto-.

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/femto