Denominations
Bitcoins can be split up in satoshi and millisatoshi.
Satoshi
The satoshi is currently the smallest unit of the bitcoin currency recorded on the block chain. It is a one hundred millionth of a single bitcoin (0.00000001 BTC). The unit has been named in collective homage to the original creator of Bitcoin, Satoshi Nakamoto.
https://en.bitcoin.it/wiki/Satoshi_(unit)
Millisatoshi
The Lightning Network has the capability to send amounts smaller than the smallest unit on the Layer-1 blockchain. The smallest unit on Bitcoin is 1 satoshi but the Lightning Network can handle millisatoshis with 1000 millisatoshis being 1 satoshi.
Millisatoshis is abbreviated as msat.
The amount is rounded down to the nearest satoshi when the channel is closed and broadcast to the blockchain to adhere to bitcoins specification.
A millisatoshi is exactly 0.00000000001 BTC. Or 100 billion millisatoshis are 1 BTC.