atomic theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The physical theory of the composition of matter stating that common materials are composed of molecules, which are composed of atoms, which are composed of nuclei and electrons. It holds that the properties of composite entities, such as their structure and behavior, arise from the properties of their components.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The theory that all gross matter is composed of atoms.
• noun physics Any of several theories that explain the structure of the atom, and of subatomic particles.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles
• noun a theory of the structure of the atom