What is the name of the hypothesis that the Moon formed from a collision between Earth and a Mars-sized body?

The hypothesis that the Moon formed from a collision between Earth and a Mars-sized body is the giant-impact hypothesis.

In this model, a protoplanet commonly called Theia struck the young Earth roughly 4.5 billion years ago. The impact blasted rock and mantle material into orbit; that debris gradually gathered into the Moon. Theia’s name comes from Greek mythology, where Theia was the mother of Selene, the Moon goddess.

The idea helps explain several lunar features: the Moon has a small iron core, contains relatively little volatile material, and shares unusually similar isotope ratios with Earth. The collision also accounts for the high angular momentum of the Earth–Moon system and the evidence that the early Moon was extensively molten.

Older alternatives include fission, in which the Moon spun out of Earth; capture, in which Earth gravitationally acquired a pre-existing body; and co-accretion, in which both bodies formed together. The giant-impact model is favored, although researchers still debate the exact impact conditions and how the debris assembled into one Moon.