Float

A company's total share count and the number of shares genuinely circulating are different things. Founders, executives, and early backers often hold large blocks that are locked up or simply never sold. Subtracting those leaves the float, which is what is really changing hands day to day.

The gap matters most for newly listed companies, where insiders may hold the large majority of the stock and only a small slice was sold to the public. It also matters for how a price behaves: when relatively few shares are available, the same amount of buying or selling moves the price further.

Most screeners and index providers work with full market cap, while free float is what affects how easily shares change hands. The two answer different questions and are both worth knowing which one you are looking at.