The number of bitcoins (BTC) that have been mined to this date. They take into account all bitcoins that were ever created. It includes those originally mined by Satoshi Nakamoto, the founder of Bitcoin, which are frozen spent or transferred after their creation. Some of the circulating supply is actually not circulating at all because the original owners have lost the cryptographic keys necessary to spend them. Some have been stolen and the thieves are still not able to spend them and are therefore locked.
According to research performed by coinmetrics.io, over 1,500,000 coins are assumed to be lost and over 197 thousand have been stolen and not reused, so they estimate the actual liquid supply to be around 16 million.