Bitcoins can be split up in satoshi and millisatoshi.
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.
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.