What's a Satoshi?

A Satoshi is the smallest fraction of a Bitcoin that can currently be sent: 0.00000001 BTC, that is, a hundredth of a millionth BTC.

However it's possible that in the future the protocol will be updated to allow further and smaller subdivisions, should they be neededand required by the community.

Examples of units of BTC

1 BTC = 1,000 mBTC (millibitcoin)
1 BTC = 1,000,000 μBTC (microbitcoin)
1 BTC = 100,000,000 Satoshis
1 mBTC = 100,000 Satoshis
1 μBTC (microbitcoin) = 100 Satoshis