Uniformitarianism is the idea that the same physical laws of today have always operated. It was the centerpiece of James Hutton's 1795 geology book Theory of the Earth, with proofs and illustrations.
In this work Hutton proposed that the causes acting on the world today also acted in the past. This means that the rocks of the Earth were formed by processes which we can see operating today. These processes, though very slow (gradualism), made the world what it is today. The idea is part of a wider philosophy called the philosophy of naturalism.
The term 'uniformitarianism' was invented by William Whewell in 1837 to describe Hutton's basic idea. This idea is the foundation of modern geology.
Hutton's ideas were popularised by John Playfair in 1802 in Illustrations of the Huttonian theory of the Earth, and by Charles Lyell in his Principles of geology (1830 to 1833). The phrase "the present is the key to the past" is Lyell's most famous quotation.
Uniformitarianism in geology is opposed by catastrophism, the idea that major changes in the Earth take place through sudden and violent events.