MerriamWebster online defines theory as: the analysis of a set of facts in their relation to one another; abstract thought; the general or abstract principles of a body of fact, a science, or an art; a belief, policy, or procedure proposed or followed as the basis of action; a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena; a hypothesis assumed for the sake of argument or investigation.
Late Latin theoria, from Greek theōria, from theōrein. First known use 1592. Synonyms: hypothesis, proposition, supposition, thesis.
From theory comes theoretical. And theoretical is: relating to what is possible or imagined rather than to what is known to be true or real; relating to the general principles or ideas of a subject rather than the practical uses of those ideas.
For Americans theory is only as good as it explains what is known to be true, and has practical use.
