(chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles; "the ancient Greek philosophers Democritus and Epicurus held atomic theories of the universe"
The doctrine or theory that reality is fundamentally material and consisting of simple and unchanging particles called atoms.
the theory, as set forth by philosophers such as Democritus, that physical objects consist of minute, indivisible particles moving in a void.
The doctrine that all things are composed of ultimate, indivisible atoms of matter endowed with motion. These ultimate particles are the enduring basis of all reality. In the modern form of this philosophy, atoms have been superseded by fundamental subatomic particles.
In natural philosophy, atomism is the theory that all the objects in the universe are composed of very small, indestructible building blocks - atoms. Or, stated in other words, that all of reality is made of indivisible basic building blocks. The word atomism derives from the ancient Greek word atomos which means "that which cannot be cut into smaller pieces".