A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested and verified in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluation of results.
It really doesn't get much clearer than that
Someone who is using the word theory for scientific theory is probably scientifically illiterate. If not, then willfully ignorant.
But that is the problem with science. Today's theories and facts can become wrong tomorrow. So saying today, "evolution is a testable theory with evidence to back it up" doesn't mean it's going to be true tomorrow. So, people are right to doubt it today if there's no guarantees it's gonna hold up in the future.
That's why if you went to school in the 80's and 90's Pluto was a planet. Nowadays, you would be wrong on your science test if you said it was a planet. So, was it right back then or is it right today? Take a science test from the 80's. "Pluto is a planet." Correct answer. Today: "Pluto is a planet." Wrong answer.
See the problem?
Ever watch a murder mystery on TV? Or read one in a book?
You know the story--a body is discovered, and an investigator is sent in to investigate. Perhaps early in the story, he suspects the victim's wife committed the murder. But he can't just declare the job done--he has to gather evidence that she had the means, motive, and opportunity.
So after some questioning, it turns out the wife has an airtight alibi--she was out of town the night of the murder. So the investigator searches for more evidence, and concludes that the murderer was the man's best friend.
More evidence-gathering ensues, and the best friend was at home with his family. So he's off the hook.
And so on. Finally, after more gathering of evidence, the investigator concludes that it was an angry neighbor who killed the victim; the neighbor confesses, and justice is served.
And that's how it should be. Had the investigator arrested the wife simply because she was his first suspect, then an innocent woman would have gone to jail, or even to Death Row. By gathering more evidence, the investigator was able to refine his conclusion and make an accurate judgment.
For some people, to "refine his conclusion" looks suspiciously like "changing his mind willy-nilly." But of course they're not the same at all. With more evidence comes better judgments.
And so it is with science. A century ago, when Pluto was discovered, we had no evidence of the Kuiper Belt. But we've gathered more evidence, and now we know that Pluto is not one of nine objects, but one of
millions. Obviously a new category of solar system objects was needed. We did the same thing back in the nineteenth century when astronomers realized that Ceres was not one of several planets but was in fact one of millions of objects. And so, we developed the concept of 'asteroid' and at a stroke, the number of identified planets
fell from twenty-three down to eight. (Boy, were people upset about that!)
And that's how it should be. Science is not dogmatic--it is always open to new evidence, which makes for better answers. Facts do not become 'wrong'; they become improved. Isn't that great?