The Collaborative International Dictionary of English v.0.48:

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.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

string theory \string" the`o*ry\ (str[i^]ng" th[=e]`[-o]*r[y^]),
   n. (Physics)
   A mathematical theory for describing the properties of
   fundamental particles, which represents the particles as
   one-dimensional string-like objects, which exist in the
   normal four dimensions of space-time plus additional
   dimensions, the total dimensions being ten, eleven, or
   twenty-six depending on the version of the theory. The
   properties of fundamental particles in string theory and
   their manner of interaction with each other depend upon the
   modes of vibration of the strings[17]. The attractiveness of
   this theory rests in part on its ability to provide a unified
   treatment of gravity as well as the three other basic forces
   of nature, in a manner consistent with quantum mechanics. The
   great difficulty of doing the calculations required by the
   theory, however, has thus far (1999) made it impossible to
   calculate the observable properties, such as the mass, of
   known particles, such as the electron, proton, mesons,
   quarks, and neutron; thus there is as yet no experimental
   verification for the theory. The most popular version of the
   theory depends on a mathematical property called
   supersymmetry, and the theory derived form this principle is
   properly called superstring theory, a term which is often
   used interchangeably with string theory. See also string[17].
   [PJC]