Circulating supply
In short Circulating supply is the number of a token's units that are in public hands and free to trade, excluding locked, unvested, reserved, or burned tokens.
Circulating supply is the number of a token's units that are actually in public hands and free to trade. It leaves out tokens that are locked, unvested, held in reserve by the team, or sent to a burn address. If a token has a billion units in existence but only 200 million are tradable, its circulating supply is 200 million.
There is no single on-chain field for it. Token standards like ERC-20 and SPL only track total supply, so circulating supply is a number someone calculates: total supply minus the wallets and contracts they decide to exclude. Data sites often take that figure from the project itself, or from a manually maintained list of excluded wallets, which means it can be stale or simply wrong.
It matters because market cap is price times circulating supply, so every ranking and screener built on market cap depends on it. A small circulating supply against a large total supply means most of the tokens have not reached the market yet. As locked tokens unlock and get sold, that new supply can push the price down even if demand stays flat. A thin circulating supply also makes the price easy to move with modest buying, which cuts both ways.
Check it yourself before trusting it. Open the token on a block explorer, look at the top holders, and see which large wallets are labeled as team, treasury, or locked contracts. Compare the listed circulating supply with what the holder list suggests. Watch for projects that exclude wallets in a way that makes market cap look smaller or larger than it really is. Then read the vesting schedule and compare market cap with fully diluted valuation to see how much dilution is still coming.