Less than one share of stock.
You cannot buy and sell fractional shares; they are only created by stock dividends and DRIPs. Stock dividends are when dividends are given in stock. DRIPs are Dividend Reinvestment Plans, in which any dividend is automatically reinvested in the company's stock. In either case, you very often may have a dividend that does not divide evenly into the stock price, making it necessary to use fractions of a stock.
Stock certificates are not generally issued for fractional shares, so these fractions are usually paid off in cash or held by the DRIP, which can add together the fractional stocks from multiple investors. In some rare cases a company will issue a scrip, which is basically a fractional stock certificate.