noun
1. : atomism 1
2. also atomic hypothesis : a theory of the nature of matter: all material substances are composed of minute particles or atoms of a comparatively small number of kinds, all the atoms of the same kind being uniform in size, weight, and other properties
3. : any of several theories of the structure of the atom, most modern ones, 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 — compare thomson's hypothesis , rutherford atom , lewis-langmuir theory , bohr theory , schrödinger atom