The Collaborative International Dictionary
Uniformitarianism \U`ni*form`i*ta"ri*an*ism\, n. (Geol.) The uniformitarian doctrine.
Wiktionary
n. (context chiefly geology English) The scientific principle that natural processes operated in the past in the same way and at the same rates that they operate today. (from 19th c.)
Wikipedia
Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. It has included the gradualistic concept that "the present is the key to the past" and is functioning at the same rates. Uniformitarianism has been a key first principle of geology and virtually all fields of science, but naturalism's modern geologists, while accepting that geology has occurred across deep time, no longer hold to a strict gradualism.
Uniformitarianism, coined by William Whewell, was originally proposed in contrast to catastrophism by British naturalists in the late 18th century, starting with the work of the Scottish geologist James Hutton. Hutton's work was later refined by John Playfair and popularised by Charles Lyell's Principles of Geology in 1830.
Usage examples of "uniformitarianism".
The antitheses of uniformitarianism and evolution are flood geology and creationism.
Royal Society of Edinburgh, advancing the idea of uniformitarianism in geology.
This was uniformitarianism applied to biology as well as geology and, once again, it was nothing like Genesis.
Uniformitarianism will crumble away when we present conclusive proof that modern landforms were chiefly shaped by the waters of the Noachian deluge.