The word theory is used a lot. However, among the lay population there is a fundamental misunderstanding or, in some instances, a purposeful misrepresentation of what the scientific meaning of word "theory" is. Does the misunderstanding and the misrepresentation of the word theory matter? The answer is an emphatic yes.
While there are many similar scientific definitions of "theory," they are all something like the following. One definition of a scientific theory is, "a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena" (Dictionary.com). Another scientific definition is, "A systematically organized body of knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena" (American Heritage Dictionary). An example is Einstein's Theory of Relativity. Other examples are Classical Mechanics Theory and Quantum Mechanics Theory. Each of these has been tested countless times and used to successfully explain and predict a vast array of physical phenomena.
When scientists use the word theory, they do not mean that there is perfect knowledge about a subject that is immutable and not subject to change or modification. Classical Mechanics Theory and Quantum Theory are excellent examples of how science evolves, but in spite of advances, the old "wrong" theory can still be right. Classical mechanics is used to describe phenomena associated with macroscopic objects. Classical mechanics is ideal for describing the flight of baseballs, landing a space craft on Mars, or designing bridges and airplanes. Classical mechanics works so well that Boeing's new 787 Dreamliner was designed completely on a computer. Classical mechanics can describe the airflow over the wings, and therefore the lift that makes a plane fly. It can also describe the properties of materials, and the stresses and strains that flight will have on them. Therefore the wings can be designed without fear that they will break off during the first flight.
However, by the end of the 19th century, because of new experiments it became obvious that classical mechanics could not explain many new phenomena that were being discovered. These all had to do with exceedingly small things, atoms, molecules, electrons, protons, etc. A new theory, Quantum Theory, was developed beginning in 1900. Quantum Theory now can successfully describe the properties of atoms and molecules. Quantum descriptions are necessary to understand the semiconductors that comprise our computers, why things have color, and why carbon dioxide is a green house gas. In some sense, Classical Mechanics Theory is wrong. It is the limiting case of Quantum Theory when you proceed from the very small (atoms and molecules) to the macroscopic (baseballs and airplanes). However, we don't use Quantum Theory to design bridges. Classical Theory still works exceptionally well in its realm of macroscopic objects. So Classical Theory is not wrong for the objects it was developed to describe. It is just incomplete in the sense that it does not describe everything.
This is a key point. Scientific knowledge increases. New things are discovered and understood. But the science that was correct and worked prior to new discoveries is still correct and works in the sphere in which it is applicable. So as science advances, established theory stays and can be improved, but it can also become only part of the story.
The scientific meaning of theory is in sharp contrast to the commonly used non-scientific term. In the non-scientific context, theory is used to mean "an untested idea or opinion." Notice that this is essentially the exact opposite of the scientific meaning of the word. When someone says he "doesn't believe in the Theory of Evolution," he is saying that he doesn't believe in the "untested idea or opinion" that is the Theory of Evolution. Or perhaps someone says that the idea that green house gases are contributors to global warming and climate change is "only a theory." This statement is meant to imply the science behind our understanding of the causes of global warming is based on "untested ideas or just opinion."
The people who make such statements about the Theory of Evolution or our understanding of the causes of climate change are either ignorant or dissembling. They use the non-scientific definition of the word theory to obfuscate for the non-scientist our true level of understanding of important scientific issues and problems. When a body of tested scientific knowledge is said to be "only a theory," the scientific meaning of the word is perverted with the effect of trivializing issues that may have life and death consequences for the entire world.