From the American Heritage Dictionary:
the·o·ry n. pl. the·o·ries
From the National Academy Press:
Is evolution a fact or a theory?
The theory of evolution explains how life on earth has changed. In scientific terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses. Biological evolution is the best scientific explanation we have for the enormous range of observations about the living world.
Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.
Why isn't evolution called a law?
Laws are generalizations that describe phenomena, whereas theories explain phenomena. For example, the laws of thermodynamics describe what will happen under certain circumstances; thermodynamics theories explain why these events occur.
Laws, like facts and theories, can change with better data. But theories do not develop into laws with the accumulation of evidence. Rather, theories are the goal of science.
From the Talk Origins site:
"Evolution is a Fact and a Theory"
Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts.
Answers in Genesis, a web site which promotes Creationism, has a section on arguments that creationists shouldn't use. Whilst they correctly direct people not to use the "just a theory" argument, their alternative is no better:
"Evolution is just a theory."
What people usually mean when they say this is "Evolution is not proven fact, so it should not be promoted dogmatically." Therefore people should say that! The problem with using the word "theory" in this case is that scientists use it to mean a well-substantiated explanation of data. This includes well-known theories such as Einstein's Theory of Relativity and Newton's Theory of Gravity, as well as lesser-known ones such as the Debye-Hückel Theory of electrolyte solutions. It would be better to say that particles-to-people evolution is an unsubstantiated hypothesis or conjecture.
From the PBS series on evolution:
When we use the word "theory" in everyday life, we usually mean an idea or a guess, but the word has a much different meaning in science. This video examines the vocabulary essential for understanding the nature of science and evolution and illustrates how evolution is a powerful, well-supported scientific explanation for the relatedness of all life.
The book The Top 10 Myths About Evolution, by Cameron M. Smith and Charles Sullivan, has a chapter entitled "Myth Two: It's Just a Theory":
...calling evolution "just a theory" involves a misunderstanding of what a scientific theory is. Evolution is a fact, and the three main processes that make up evolution - replication, variation, and selection - are observable and undeniable.