A share buyback is a corporate action in which a company purchases its own shares from existing stockholders, reducing the outstanding share count. Also called share repurchase or stock buyback, the repurchased shares either become treasury stock or are canceled, used at public companies as a capital-return mechanism and at private companies as an employee/investor liquidity mechanism via tender offers. It is the corporate analog of an individual stockholder selling shares, with the company itself as the buyer.
The two main contexts:
Public-company buybacks: