Theory

Most educated people have some idea about what a theory is. They recognize one (good or bad) when they hear or read about one, but the concept is difficult to define.

The following is due to Nikolai Joyce.

A theory is a set of interrelated ideas derived from research, observation, experiments, or a self consistent set of axioms, used to organize data, explain phenomena, and predict outcomes.
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Avoid the following horrors when you write. Do not start a discussion with ...

In theory ...

Theoretically ...

... or some contorted equivalent.