Uniformitarianism is a school of scientific philosophical thought that states that the modern laws of nature that we observe today have always been present since the formation of the universe and are also found throughout the universe (in other words in the parts that are unobservable to us). This can be more simply expressed with the phrase "the past is present." In other words, we can deduce that natural forces in effect today have always been present, thus we can use the present as a starting point into the past.
While this can be mostly readily accepted in areas like chemistry (atom structure remaining unchanged since formation) and physics (light and energy properties unchanging over time), in geology it is not as simple. For example, have the Earth's tectonic plates always moved, or did that phenomenon start after a certain point of maturation? Geological uniformitarianism was popularized by 19th century British geologist Charles Lyell. He was a major proponent of the theory. The opposing theory is called catastrophism, which holds that periodic catastrophic events formed the Earth as we know it, with long periods of calm between events.
Modern geology tends to favor a mix of the two theories. They largely believe in uniformitarianism (though not quite to the Lyellian degree), but period catastrophic events have upset the careful balance and had dramatic impact.
This is a key principle of geology and is the assumptions where the same natural laws and processes that are in the universe now have always operated the universe and will remain the same anywhere and anywhere within the universe. Thus, the saying "the present is the key to the past" can relate.
Uniformitarianism is a thought process governed primarily by the belief that the forces at work on Earth today have always been there. For instance: volcanic activity is a force that shapes our planet now, therefore it must have had a role in shapping our planet in the past. Usually there is evidence to support this theory. Using our example, there are volcanic vents that are millions of years old.
Uniformitarianism comes from the word uniform, meaning orderly and of the same state. Thus this is a topic of debate in geology as geologists may not agree on matters such as the constant force that is responsible for shifting the textonic plates since billions of years ago.
Uniformitarianism in geology is basically the theory that the changes in the earths crust during geological history are the result of continuous processes.
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.