Denominations
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 abbreviate with 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 Millisatoshis is the 0.00000000001-th part of a Bitcoin. Or 100 billion Millisatoshis are 1 BTC.