Final answer:
Uniformitarianism is the idea that past geological changes occurred gradually through the same processes that operate in the present.
Step-by-step explanation:
Uniformitarianism is the idea that the same natural laws and processes that operate in the present have always operated in the past. This means that geological change occurs gradually over long periods of time rather than through sudden, catastrophic events. Observations consistent with uniformitarianism include the formation of valleys by rivers wearing through rock and the slow movement of wind and rain shaping mountains, lakes, and rivers. Inferences consistent with uniformitarianism include the belief that Earth must be much older than previously suspected due to the slow-acting geological forces that produce significant change over time.