[Greek mesos, middle.]
Mesopotamia (Greek potamos, river) is an ancient region of SW Asia in present-day Iraq, lying between the Rivers Tigris and Euphrates; Meso-America is the central region of America, from central Mexico to Nicaragua. The mesoderm is the middle layer of an embryo in early development. The Mesolithic (Greek lithos, stone) is the middle stone age, between the Palaeolithic and Neolithic, while the Mesozoic (Greek zōion, animal) is the era between the Palaeozoic and Cenozoic eras, comprising the Triassic, Jurassic, and Cretaceous periods. A mesomorph (Greek morphē, form) is a person whose build is compact and muscular, intermediate between an ectomorph and an endomorph. A meson is a subatomic particle which is intermediate in mass between an electron and a proton.