Theory

A theory, in lay terms, is an unproved or unprovable guess similar in definition to a scientific hypothesis. In scientific terms it is "an explanation of a set of related observations or events based upon proven hypotheses and verified multiple times by detached groups of researchers." Thus it is the end result of the scientific method.

Scientific theories are used to make predictions and to advance technology. They are falsifiable and can be modified or, rarely, replaced based on new evidence. However, they should not be treated as proof and never become laws, which are starting points or descriptive facts about natural phenomena. Examples of scientific theories include the theory of relativity, atomic theory, and quantum theory.

Theory of evolution
Evolution is a scientific theory. The modifiable tenets of the theory of evolution can be identified as follows:
 * Individuals in a population have some variation in characters.
 * Some characters are heritable.
 * More offspring are produced in a population than can survive.
 * The offspring with the most advantageous characters tend to survive and reproduce (natural selection).
 * Cumulative changes in populations over time result in new species.

Theory of creationism
The defense of creationism depends largely on attempts at disproving evolution and even on altering the definitions of theory and science. Even its proponents admit that its predictions are unfalsifiable and that its tenets are difficult to define. Insofar as creationism has tenets they are assertions based on revelation. The Institute for Creation Research lists tenets of both "scientific" and "biblical" creationism and further indicates that they were "incorporated permanently in its By-Laws," thus admitting their immutability.

Note that "theories" based on religion are not scientific. They are not falsifiable, are not changeable based on new evidence, and do not conform to the scientific method.