A unanimous shareholders' agreement (USA) is a contract among all of a corporation's shareholders that governs how they will deal with one another and with the corporation. It sets out what shareholders can and cannot do, provides dispute-resolution mechanisms, fixes the votes required for important decisions, and establishes clear rules for transferring shares or offering them as collateral.
A USA can also include protective provisions such as "drag-along" rights, which can compel minority shareholders to sell with the majority on a takeover, and "piggyback" or tag-along rights, which let shareholders join a sale another shareholder is making to a third party. It is widely regarded as an essential document for any corporation with more than one shareholder, and it is best negotiated and signed while the business is new and the shareholders are still on good terms.