Making sure to avoid any confusion about what "scientific theory" in context of evolution means, Richard Dawkins in The Greatest Show on Earth: The Evidence for Evolution wrote:
Evolution is a fact. Beyond reasonable doubt, beyond serious doubt, beyond sane, informed, intelligent doubt, beyond doubt evolution is a fact.
- Richard Dawkins
What I'm guessing he meant is: evolution did happen and it's still happening. This is proven and solid. Evolution also uses and incorporates other facts that support the conclusion that evolution did happen. In this sense, evolution is a scientific theory and it is supported by facts. And of course, it is true and should be universally accepted as the truth. Creationists would not agree with this, but I would say this doesn't really matter to educated people.
But still, a lot of people say things like "that's only a theory", "why should we base our lives on a theory?, etc when they're talking about evolution, for example. Yeah, like a belief in a sky-wizard is something to base your life on? But I digress - the point of this brief post is just to share a understanding of what scientific theory is.
First thing you'll notice about scientific theories is - there's really PLENTY of evidence for them. This very fact makes them different from what we usually mean when we say "Theory" in normal speech.
Triceratops mounted skeleton at Los Angeles Museum of Natural History, Los Angeles, United States of America, by Allie_Caulfield Derivative: User:MathKnight - File:LA-Triceratops mount-1.jpg (by Allie_Caulfield), CC BY-SA 3.0, Link
By using words such as "FACT", instead of "theory" Dawkins does not confuse people that might not be familiar with scientific meaning of the word "theory" and sometimes I wish other scientists would do the same when talking to the public, just to hammer the point more clearly. If you read Dawkins' quote again, there's no second thought about what he meant evolution is.
Let's not get tangled in words too much, but get right to the point about evolution and other well established scientific theories. So, what does actually mean when a scientist says something is a "scientific theory"?
Well, if you simply ask Wikipedia, it will tell you:
A scientific theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed, preferably using a written, pre-defined, protocol of observations and experiments. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge.
Basically, when scientists use word "theory", they are talking about something completely different than what general public understands under "theory". When you use word "theory" in everyday speech, you actually mean "hypothesis" or "conjecture", something not tested or proven. When scientists use this word, they mean almost the same thing Dawkins meant: That what you and I mean by "fact" in normal speech.
To make an additional point, here are some scientific theories:
- Special Relativity
- General Relativity
- Plate Tectonics
- Evolution by natural selection
- Information theory
All these theories have stood the test of time, fought-off challenges, have practical effects and are well-proven pieces of knowledge humanity has. So, don't be fooled by the words and cheap arguments that something is "only a theory". But to be critical and do try and see for yourself. A good rule of thumb would be: if there's evidence for something, it's on it's way to being true and if an idea doesn't have any evidence but requires "faith" or "trust" from you, or only focuses on trying to find flaws in an existing idea, it most likely isn't true.