What is a satoshi (sat)?
The smallest unit of bitcoin: one hundred-millionth of a bitcoin. 100,000,000 sats make 1 BTC.
Bitcoin amounts are recorded in whole satoshis, named after Bitcoin's creator, Satoshi Nakamoto. People usually call them sats.
Small amounts and fees are easier to read in sats: 0.00021 BTC is 21,000 sats.