Why is the theory of evolution called a theory?
Many types of theories exist but they are not what we are usually speaking about in science. For example music theory.
The term theory in science has strict rules.
A scientific theory is an explanation of some aspect of the natural world that is acquired through the scientific method.
It is tested and tested again and again and must always show the same results.
Many people will say that is "just a theory" but in science that is not what is meant. A theory is the very last step in proving or disproving a hypothesis to a scientific question.
You may know about the theory of gravity or perhaps the theory of relativity. The theory of evolution falls into this category as well.