[From Greek atomos: indivisible.]
(metaphysics) Any metaphysical or cosmological idea that reality is fundamentally made up only of extremely small, indivisible particles. This view was widespread among ancient Greek philosophers such as Democritus (460?-362? BCE) as well as in the school of Epicureanism, and was revived in modern times by scientists and philosophers such as the 17th-century Epicurean Pierre Gassendi.