##### Wiktionary

**yocto-**

pre. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10^{−24}. Symbol: y

##### Wikipedia

**Yocto-**

** Yocto-** (symbol

**y**) is a unit prefix in the metric system denoting a factor of 10 or .

It was adopted in 1991 by the General Conference on Weights and Measures. It comes from the Latin/Greek , meaning "eight", because it is equal to 1000. *Yocto-* is the smallest official SI prefix.

It can be used to state a subatomic particle's mass, for example:

::{| |- | Rest mass of electron:|| |- |Rest mass of proton: || |- |Rest mass of neutron: || |- | Atomic mass unit: || |}