What is Satoshi (sat)?
The smallest unit of bitcoin: one hundred-millionth of a BTC. There are 100,000,000 satoshis in one bitcoin.
Bitcoin has no sub-unit smaller than a satoshi, and the protocol works in satoshis internally; "BTC" is a display convention. Amounts that look awkward in decimals are usually clean in sats.
Fee rates are quoted in satoshis per virtual byte, which is why understanding the unit matters practically and not just semantically.
Related terms
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This definition is part of the Bitcoin Logical glossary. For the fuller explanation, start with Learn, or see the live numbers on The Bitcoin Signal.