Definition of: uniformitarianism

(y′nə·fôr′mə·târ′ē·ən·iz′əm) noun Geol.
The doctrine that essential uniformity in causes and effects, forces and phenomena, has prevailed in all ages of the world's physical history, and that the activities of the past were similar in mode and intensity to those of the present: opposed to catastrophism.
—u′ni·form′i·tar′i·an adjective & noun