Float is the number of a company's shares that are available for public trading. It is also called the free float, and it leaves out shares that are locked up or closely held.
Float is calculated by subtracting restricted shares and insider-held shares from total shares outstanding. The shares that remain are the ones that can change hands freely on the open market, so a smaller float can mean thinner liquidity and sharper price moves.
Free float is not the same as shares outstanding. Shares outstanding counts every share a company has issued, including those held by insiders, founders, and the company itself, while the float counts only the portion the public can actually trade. The word float also has a separate banking sense, where it means money in transit between accounts before a payment clears.
A company has 100 million shares outstanding.
Insiders and restricted shareholders hold 30 million of them.
The float is:
100 million - 30 million = 70 million shares
That leaves 70 million shares available for public trading.