Is evolution a theory? why?

Yes.

It's actually specifically called "The Theory of Evolution."

A theory can be defined as a statement of an idea meant to try to explain something.

The theory of evolution tries to explain how species change gradually over time.

The most commonly known example of this is when Charles Darwin went to the Galapagos Islands and observed how the different islands all had finches, but each finch type was unique to the island. This prompted him to propose the theory of evolution, to try to explain how the finches were all finches but all different at the same time, and to explain how this could possibly be.