An abbreviation for satoshi; the smallest unit of the bitcoin. A satoshi (lowercase first letter) is the smallest denomination of a bitcoin (BTC), equivalent to 100 millionth of the cryptocurrency. In other words, 100 million satoshi is equal to 1 bitcoin:

1 satoshi = 0.00000001 BTC.

A satoshi gives the bitcoin its unparalleled payment flexibility as a digital cryptocurrency.

It is called so after the first name of the assumed inventor and technical author of bitcoin, Satoshi Nakamoto.