The compound "three-body" is a modern scientific formation, directly combining the cardinal number "three" with the noun "body." The word "three" has a deep Indo-European lineage, traceable to the Proto-Indo-European root trei-, which signifies the quantity. "Body," from Old English bodig, originally referred to the trunk of a person or animal but broadened to mean any material object or collection of matter. In scientific terminology, this compound transparently denotes a system of three distinct physical entities, most famously applied to the classical "three-body problem" in celestial mechanics, which studies the motion of three gravitationally interacting masses like stars or planets.

Structure: three (number) + body (physical object) - three: From Old English þrīe, of Germanic origin; related to Dutch drie and German drei, from an Indo-European root shared by Latin tres and Greek treis. Functions as a numerical quantifier. - body: From Old English bodig, of unknown origin. Functions as a noun denoting a physical entity or mass.