The American Heritage® Dictionary of the English Language, 4th Edition
n. The theory that all geologic phenomena may be explained as the result of existing forces having operated uniformly from the origin of the earth to the present time.
the GNU version of the Collaborative International Dictionary of English
n. The uniformitarian doctrine.
The Century Dictionary and Cyclopedia
n. The theory advocated by uniformitarians: the opposite of catastrophism. See catastrophe, 3, and catastrophism.
Word Usage
"The term uniformitarianism was first used in 1832 by William Whewell, a University of Cambridge scholar, to present an alternative explanation for the origin of the Earth."