Biological evolution is a change in the genetic characteristics of a population over time. That this happens is a fact. Biological evolution also refers to the common descent of living organisms from shared ancestors. The evidence for historical evolution -- genetic, fossil, anatomical, etc. -- is so overwhelming that it is also considered a fact. The theory of evolution describes the mechanisms that cause evolution. So evolution is both a fact and a theory. See the Evolution is a Fact and a Theory FAQ, the Introduction to Evolutionary Biology FAQ and the Five Major Misconceptions about Evolution FAQ: Evolution is Only a theory.