Merriam Webster
noun. evo·lu·tion \ˌe-və-ˈlü-shən
The process by which changes in plants and animals happen over time.

Urban Dictionary
The idea that organisms change over time, and that this change occurs through natural selection, which allows favorable traits to be passed along through generations. It has been referred to as a theory, which in science means it is an idea that has not been disproven. Many scientists are moving towards calling it the Law of Evolution, similar to the Law of Gravity, in order to help reduce frivolous claims that it is “only a theory” by people ignorant of what a scientific theory entails.