unilaterality

Unilaterality, or unilateral nature, refers to characteristics or actions that involve only one side, party, or aspect, without simultaneity on the opposite side or without mutual agreement. The term derives from Latin unus 'one' and latus 'side.' In common usage, it contrasts with bilateral (two sides) and multilateral (many sides).

In international relations and law, unilateral action is undertaken by one state or actor without formal consent or cooperation of others. Unilateralism is the philosophy or practice of preferring or pursuing such actions. Examples include unilateral recognition of states, unilateral sanctions, or military interventions. Critics argue it can undermine multilateral institutions and stability; supporters argue it offers swift responses to threats or opportunities.

In contract law, a unilateral contract is formed when one party makes a promise in exchange for the other's performance. The contract is not formed until performance occurs. Common examples include reward offers or contests. Unilateral contracts differ from bilateral contracts, where both sides exchange promises.

In medicine, unilaterality refers to a condition affecting one side of the body. Examples include unilateral facial nerve palsy, unilateral deafness, unilateral weakness, or unilateral neglect after brain injury. In anatomy and neurology, it is useful for localizing pathology to one hemisphere or one side of the body.