The Collaborative International Dictionary of English v.0.48:

Uniformitarian \U`ni*form`i*ta"ri*an\, a. (Geol.)
   Of, pertaining to, or designating, the view or doctrine that
   existing causes, acting in the same manner and with
   essentially the same intensity as at the present time, are
   sufficient to account for all geological changes.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Uniformitarian \U`ni*form`i*ta"ri*an\, n. (Geol.)
   One who accepts uniformitarianism, or the uniformitarian
   doctrine.
   [1913 Webster]