Merriam-Webster Collegiate Dictionary - atomic theory

 noun  Date: 1814  1. a theory of the nature of matter: all material substances are composed of minute particles or atoms of a comparatively small number of kinds and all the atoms of the same kind are uniform in size, weight, and other properties  2. any of several theories of the structure of the atom; especially one based on experimentation and theoretical considerations holding that the atom is composed essentially of a small positively charged comparatively heavy nucleus surrounded by a comparatively large arrangement of electrons