A share buyback (repurchase) is a company using its own cash to buy back its own shares from the market. The shares bought are either cancelled or held as treasury stock (held in the company's own name, carrying no votes and no dividends). Either way they leave the shares outstanding count that EPS is divided by. In the US, open-market buybacks typically run inside the Rule 10b-18 safe harbour, which sets price, volume, timing and single-broker limits so the company is not accused of manipulating its own stock.