Ka·lu·za-Klein theory 
(kə-l′zə-klīn)
(kə-l′zə-klīn)n.
1. 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.
2. Any of various physical theories introducing dimensions additional to the four dimensions of space and time.
[After Theodor Kaluza (1885-1954), German mathematician and physicist, and Oskar Klein (1894-1977), Swedish physicist.]