So, let’s recall our definitions.
Merriam-Webster says that a theory is “a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena”.
How does evolutionary theory fit this bill? It provides a body of principles to explain the phenomena related to the origin and diversification of life. Scientists generally affirm these principles, even though some disagree on certain details–such as punctuated equilibrium.
AHD, 4E expands on M-W: Evolution is a “set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.” The first part, up to the comma, I’ve already covered: Evolutionary theory is a set of principles, widely accepted by scientists, that explains a group of phenomena related to the origins and diversification of life. But AHD offers two points that M-W misses: repeated testing and predictions about natural phenomena.
Evolution has been repeatedly tested. Take just one example: bacterial resistance to antibiotics. We know that certain diseases are now harder to treat with drugs because the bacteria that cause those diseases have evolved defenses against antibiotics.
Evolutionary theory makes predictions about natural phenomena. Using evolutionary theories, scientists can make predictions about certain species might evolve to adapt to their environments. For example, evolutionary theory predicts that an organism in a rapidly changing environment should have higher mutation rates.
So, to recap, evolution is a theory. It provides generally accepted and scientifically plausible principles to explain how life arose on Earth and how it attained its present diversity. Evolutionary theory makes predictions that scientists can test through observation and experimentation.
A theory is not an intuition, a guess, or a hunch. The theory of evolution is not four teenagers and a dog riding around in a van chasing men dressed like ghosts because Velma had a “theory” about Old Mr. Scruggs. Dismissing evolution as “just” a theory shows a dramatic misunderstanding of what a theory is.
See also “Evolution is only a theory,” at EvoWiki.org.