In debates about science, particularly about evolution, one often hears the phrase, “it’s just a theory.” This is an example of argumentum ad nauseum in that it is meant to dismiss the science of evolution by using the often repeated phrase.
So where did the “just a theory” phrase come from? Essentially it’s a substitution of the scientific use of the term “theory” with the daily use of the term “theory.”
A theory in science is an accepted principal used to explain phenomena.
Here’s what the US National Academy of Sciences has to say on the matter:
The formal scientific definition of theory is quite different from the everyday meaning of the word. It refers to a comprehensive explanation of some aspect of nature that is supported by a vast body of evidence. Many scientific theories are so well established that no new evidence is likely to alter them substantially. For example, no new evidence will demonstrate that the Earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the Earth is not divided into solid plates that have moved over geological timescales (the theory of plate tectonics). One of the most useful properties of scientific theories is that they can be used to make predictions about natural events or phenomena that have not yet been observed.