Term of the Day theory A set of assumptions, propositions, or accepted facts that attempts to provide a plausible or rational explanation of cause-and-effect (causal) relationships among a group of observed phenomenon. The word's origin (from the Greek thoros, a spectator), stresses the fact that all theories are mental models of the perceived reality. Learn more about this term Usage Example One of the most well-known theories is the Big Bang Theory, which is one plausible explanation of how the universe was formed.