Yocto (symbol y) is a unit prefix in the metric system denoting a factor of 10−24 or 0.000000000000000000000001.
It was adopted in 1991 by the General Conference on Weights and Measures. It comes from the Latin/Greek octo (οκτώ), meaning "eight", because it is equal to 1000−8. Yocto- is the smallest official SI prefix.
It can be used to state a subatomic particle's mass, for example:
References
Yocto- Wikipedia(Text) CC BY-SA