The amount of Bitcoin in circulation. Bitcoin'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's money supply is transparent and on a fixed schedule. Bitcoin's inflation rate changes with block height.
The Bitcoin 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 bitcoins will exist and no more will be created.