Taken from the wiki link iheartnikon posted:
"Evolution is a "theory" in the
scientific sense of the term "theory"; it is an established scientific model of a portion of the universe that generates propositions with observational consequences. Such a model both helps generate new research and helps us understand observed phenomena.
When scientists say "evolution is a fact", they are using one of two meanings of the word "fact". One meaning is empirical: evolution can be observed through changes in allele frequencies or traits of a population over successive generations."
There's your answer.