Opponents of evolution argue that evolution is "just a theory." What does the word theory mean in a scientific perspective?

A theory is an idea of some natural event which explains the natural event. The theory word used in the everyday language differs from the scientific theory as the theory represents a guess based on the belief system with little or no evidence whereas the scientific theory is a well-substantiated idea which is proved through the experiments and supported by the evidence.

The scientific theory, however, changes with time as the developed technology and some new experiments changes the evidence which could be against the scientific theory.