Any theory basing either moral obligation in general, or the duty of political obedience, or the justice of social institutions, on a contract, usually called a 'social contract'.
The idea goes back at least as far as Plato's Crito (c.395 BC), and contractualists (or contractarians) have also included Thomas Hobbes (1588-1679), John Locke (1632-1704), Jean Jacques Rousseau (1712-1778), and various modern writers.
The contract may be an allegedly historical one or a tacitly implied one, or an imaginary one. It may be between people who set up a sovereign, or between the people and the sovereign, or between the individual and society or the state, or between hypothetical beings in a setting making for impartiality.
J Rawls, A Theory of Justice (1972)