In scientific terms, evolution is both a theory and a fact. A fact refers to a scientific explanation that has been tested and confirmed often enough that there is no reason to keep looking for additional examples. Scientists accept evolution as a fact due to overwhelming evidence supporting it.
As the National Academy of Science points out, the word "theory" in science can refer to an explanation that is supported by vast evidence. Scientists are confident that no new evidence is likely to overturn the theory of evolution due to the many observations and experiments supporting it. Scientists typically no longer question whether evolution occurred but instead study the mechanics of evolution and related subjects.