Dictionary.net

One who upholds a system or doctrine of uniformity; specifically, one who maintains that all geologic changes and phenomena are due to agencies working uniformly and uninterruptedly, as opposed to a catastrophist, who refers such changes to great occasional convulsions. The uniformitarian maintains that the influence of the agencies that we see working now, continued during all the aeons of geologic time, is sufficient to account for all the phenomena presented to us in the structure of the earth.

Of or pertaining to uniformity or the doctrine of uniformity. "The catastrophist and the uniformitarian opinions."-Whewell.