The Collaborative International Dictionary
grand unification theory
grand unified theory \grand unified theory\, grand unification theory \grand unification theory\n. (Theoretical physics) Any of a class of physics theories that attempts to explain the electroweak forces, stong force, and gravitation within a single mathematical conceptual scheme. In the 1990's string theory and superstring theory were prominent examples. Abbreviated GUT, plural GUTs.
Wiktionary
grand unification theory
n. (context physics English) A theory that unifies the electromagnetic interaction, the weak interaction, and the strong interaction.