Introduction

A very common evolution misconception is that it is just a theory.
This misconception is quite simple to explain: the layman's use of the
word "theory"
is in a different context from how scientists use the term.

Discussion

The layman's meaning of the term "theory" tends to be synonymous with
a hunch or a guess (such as "I've got a theory..."); it also seems to
imply uncertainty (as expressed in the phrase "it's a theory, not a
fact").

However, the scientific meaning of a theory has a more precise
and mechanical definition: "An extremely well-substantiated explanation
of some aspects of the natural world that incorporates facts, laws,
predictions, and tested hypotheses" [1]. Clearly,
when scientists use the term "theory of evolution", they really mean
that evolution is a large body of consistent, well-substantiated facts,
laws, predictions, and observations that describe natural phenomena;
they don't use the word to imply that evolution is yet to be proven.

There is a popular misconception that a theory must be proven,
then it becomes a fact or a law. However, a scientific theory is, most
generally defined, a large body of facts and laws, so this criticism
does not make sense.

The weakness of the "just a theory" criticism becomes obvious
because in light of other well known theories, such as Einstein's
theories of Relativity, atomic theory, germ theory, quantum theory,
electromagnetic theory, plate tectonics theory, etc. No one would
seriously imply that any of these theories are unproven and uncertain.

Quotations

Theory: In science, a well-substantiated explanation of some
aspect of the natural world that can incorporate facts, laws,
inferences, and tested hypotheses. --- US National Academy of
Sciences [2]

"Theory" means a logical, tested, well-supported explanation
for a great variety of facts. --- National Center for Science
Education, USA [3]

Theory: A scientifically testable general principle or body
of principles offered to explain observed phenomena. In scientific
usage, a theory is distinct from a hypothesis (or conjecture) that is
proposed to explain previously observed phenomena. For a hypothesis to
rise to the level of theory, it must predict the existence of new
phenomena that are subsequently observed. A theory can be overturned if
new phenomena are observed that directly contradict the theory. ---
NASA[4]

Scientific theories, like evolution and relativity and plate
tectonics, are hypotheses that have survived extensive testing and
repeated verification. Scientific theories are therefore the
best-substantiated statements that scientists can make to explain the
organization and operation of the natural world. Thus, a scientific
theory is not equal to a belief, a hunch, or an untested hypothesis. Our
understanding of Earth's development over its 4.5 billion-year history
and of life's gradual evolution has achieved the status of scientific
theory. --- American Geophysical Union [5]

A theory in science, such as the atomic theory in chemistry
and the Newtonian and relativity theories in physics, is not a
speculative hypothesis, but a coherent body of explanatory statements
supported by evidence. The theory of evolution has this status. ---
American Intitute of Biological Science [6]

Evolution is both a scientific fact and a scientific theory.
--- The Paleontological Society [7]

Where the Word "Theory" Does Not Apply

Very often, when a evolution is criticized for being "just a theory",
it is paired along side Intelligent Design theory - the
implication is clearly that, both being nothing more than "theories",
they both must be on the same level of credibility.

Of course, we know this is not true, because to say that
evolution is a scientific theory implies that it is rigorously tested
and well-substantiated.

On the other hand, Intelligent Design is not and could never
be considered a scientific theory: Intelligent Design adds no new knowledge
to the world, postulates hypothetical unfalsifiable entities, cannot be
described in terms of scientific laws or mathematics, and cannot be
falsified even in principle. It cannot be studied, tested, empirically
verified, or even have the scientific method applied to it.

Quite literally, Intelligent Design is as much of a
scientific theory as Last Thursdayism (the "theory" that the universe was
created Last Thursday with an apparent old age).

Related Terminology

It is helpful to understand the scientific meaning of words:

• Observation: Anything you see, hear, smell, taste, or touch,
  with or without special instruments, is an observation
  

• Hypothesis: a testable statement about the natural world that
  can be used to build more complex inferences and explanations.
  

• Experimentation: the process of testing a hypothesis by
  collecting data under controlled, repeatable conditions
  

• Fact: when an observation is confirmed repeatedly and by many
  independent and competent observers, it can be regarded as a fact.
  

• 
  Law: a descriptive generalization about how
  some aspect of the natural world behaves under stated circumstances.
  Very often, laws are expressed in terms of mathematical relationships
  (see the equations for Boyles Law, Ohms Law, Snells Law, etc.).
  

References and Resources

• 
  TalkOrigins - Evolution is a Fact and Theory
  


• 
  Understanding Evolution - Misconceptions - "Just a
  Theory"
  


• 
  Glossary of
  Scientific Terminology