In Wikipedia: Theory

A theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking.

Depending on the context, the results might for example include generalized explanations of how nature works.

The word has its roots in ancient Greek, but in modern use it has taken on several different related meanings.

A theory is not the same as a hypothesis. A theory provides an explanatory framework for some observation, and from the assumptions of the explanation follows a number of possible hypotheses that can be tested in order to provide support for, or challenge, the theory.

(Source: Wikipedia)