The total supply of coins/tokens of a cryptocurrency that have been issued and are in the hands of market participants.
Most cryptocurrencies have fixed maximum supply figures and emission schedules that slowly release coins/tokens into the market. This model means, at any given time, a cryptocurrency has a theoretical max supply and an actual circulating supply.
For instance, if a cryptocurrency has a max supply of 1 billion coins/tokens, but only 100 million have been released so far, the cryptocurrency's circulating supply would be 100 million.