What is the name of the theory that unifies electromagnetism and the weak nuclear force?

The theory that unifies electromagnetism and the weak nuclear force is electroweak theory. It describes these apparently different forces as two aspects of a single interaction at sufficiently high energies.

At everyday energies, electromagnetism has an effectively infinite range because it is carried by massless photons, while the weak interaction has a very short range because it is carried by massive W and Z bosons. The Higgs mechanism breaks electroweak symmetry, separating the physical photon from the massive W and Z particles.

Sheldon Glashow, Abdus Salam, and Steven Weinberg developed the modern electroweak framework. Their work earned the 1979 Nobel Prize in Physics. Experiments supported the theory through the discovery of neutral currents in 1973 and the W and Z bosons in 1983.

Electroweak theory is part of the Standard Model, but it is not a grand unified theory: the latter would also seek to combine the strong interaction with the electroweak forces. Quantum chromodynamics describes the strong force alone, while string theory is a broader speculative framework rather than the established answer to this question.