Kaluza-Klein theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A theory that seeks to unify the force of electromagnetism as described by Maxwell's equations and the force of gravity as described by general relativity, explaining each as manifestations of a single fundamental force and introducing a hypothetical fifth dimension.
• noun Any of various physical theories introducing dimensions additional to the four dimensions of space and time.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun A theory (no longer accepted) developed by physicists Theodor Kaluza and Oskar Klein that attempted to combine gravity and electromagnetism by adding a fifth dimension to our four-dimensional spacetime.
• proper noun Any of a number of theories that try to explain forces by adding extra dimensions to our spacetime.