A Satoshi is the smallest unit of Bitcoin, it is the 8th digit after the decimal point. It is therefore equal to 0.00000001 BTC. It was named after Satoshi Nakamoto, the creator of Bitcoin, its acronym is Sat. It is in a way the cents of our euros for Bitcoin.
1 Bitcoin (BTC) = 100,000,000 satoshi
100,000 satoshi = 1mBT or 0.001 BTC