| 1. | a coherent group of general propositions used as principles of explanation for a class of phenomena: Einstein's theory of relativity. |
| 2. | a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact. |
| 3. | Mathematics. a body of principles, theorems, or the like, belonging to one subject: number theory. |
| 4. | the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory. |
| 5. | a particular conception or view of something to be done or of the method of doing it; a system of rules or principles. |
| 6. | contemplation or speculation. |
| 7. | guess or conjecture. |

In science, an explanation or model that covers a substantial group of occurrences in nature and has been confirmed by a substantial number of experiments and observations. A theory is more general and better verified than a hypothesis. (See Big Bang theory, evolution, and relativity.)
theory the·o·ry (thē'ə-rē, thēr'ē)
n.
A systematically organized body of knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena.
Abstract reasoning; speculation.