any field theory, especially Einstein's, that attempts to combine the gravitational and electromagnetic fields in a single mathematical framework, thus extending the general theory of relativity.

British Dictionary definitions for unified field theory

unified field theory

noun

any theory capable of describing in one set of equations the properties of gravitational fields, electromagnetic fields, and strong and weak nuclear interactions. No satisfactory theory has yet been found

A theory that unites and explains the basic forces of nature (strong, electroweak, and gravitational forces) as manifestations of a single physical principle. No unified field theory that has been proposed so far has gained broad acceptance.