A theory or set of theories in physics which will finally explain all matter and energy interactions and manifestations. It will unify all known theories of physics: gravity, electromagnetism, and the weak and strong forces in the atom. Ideally, it would unify all the theories of all branches of science, and include the humanities, philosophy and religion.
The final theory of particle physics. It will describe the electromagnetic, weak, strong, and gravitational forces as different aspects of the same force. It will not end all of physics, however. Just because you can describe how all of the quarks, electrons, photons, etc. interact in the human body does not mean that you can describe how a person will react if he has his head smashed by a hammer. It would take too long to calculate the result of every interaction between every particle in the body. See also: strong force, electromagnetic force, gravitational force, Grand Unified Theory