A scientific theory is much different than a "theory" in common terms. While the word "theory" is generally used to mean a guess or an uncertainty, scientific theories are much more than that.
In science, an idea starts out as a hypothesis, or an educated guess as to why or how something is happening. Those hypotheses undergo a lot of experimental trials in various ways and data is collected. Over time, as more and more data is collected, a hypothesis can become a scientific theory. The more a hypothesis is supported by many different experiments and many different experimenters, the stronger the scientific theory. However, scientific theories can change as more data is collected.
The Theory of Evolution is a scientific theory. It began as a hypothesis that has been tested time after time and always supported. There is a plethora of data to back up the Theory of Evolution. Even though detractors say that it's just a "theory", the fact that it is a scientific theory means it is much closer to being a scientific truth than an educated guess.
In addition to the Theory of Evolution, other scientific theories include the Big Bang Theory, the Atomic Theory, and the Cell Theory.
