As a physical theory atomism was invented by Leucippus and Democritus in the 5th century BC, developed by Epicurus a century or so later, and revived in the 17th century.
In it matter consisted of tiny indivisible, indestructible and unchanging bits of solid stuff, differing in shape and size, and jostling each other in the void to constitute the material world. They were responsible for colors, smells, tastes, and so on (the ‘secondary’ qualities; also see: effluxes), but did not themselves have them.
In the earliest phase physical divisibility may not have been distinguished from conceptual or mathematical divisibility, despite the atoms’ having shapes. The rival ‘continuous’ theory of matter, with no void, was held by Aristotle (384-322 BC) and the Stoics, but atomism has proved more fruitful in the development of modern physical theory, despite its enormous differences.
More generally, any theory can be called atomism which analyzes a certain set of phenomena in terms of a set of (not necessarily physical) building blocks, each having a narrowly circumscribed set of properties (for example, sensations or ideas in the case of sensationalism or associationism).
Also see: logical atomism, semantic atomism