The amount of Bitcoin Core (BTC) in circulation. Bitcoin Core (BTC)'s money supply function is specified in terms of block height.
Unlike fiat currency, where the Money Supply is perpetually increasing and not exactly known, Bitcoin Core (BTC)'s money supply is transparent and on a fixed schedule. Bitcoin Core (BTC)'s inflation rate changes with block height.
The Bitcoin Core (BTC) network is designed so that Blocks are created every 10 minutes on average. When bitcoin began, 50 bitcoins were created with every Block. That Block reward halved to 25 bitcoins in 2012, and halved again to 12.5 in 2016. This reward will keep halving until it becomes zero, when a maximum of 21 million Bitcoin Core (BTC) will exist and no more will be created.
Starting in 2017, several forks of Bitcoin gained enough market attention for 24/7 trading and a measurable market cap. Bitcoin Core (BTC) and Bitcoin Cash (BCH) follow the same 21 million supply cap as outlined in the original Bitcoin Whitepaper.