Democritus of Abdera was an important Greek philosopher who lived from 460 to 370 BC. He was a disciple of Leucippus of Miletus (Greek philosopher).
He was also an astronomer and mathematician. His ideas about atomism were that all matter was made up of small particles, which he named atoms, which means undividable particles. Atoms is a Greek word where "a" means no and "toos" means piece.
It is unclear whether these atomistic ideas were his own or that of his master Leucippus. For Democritus, the universe was infinite, where there were many other similar worlds like ours. There are records that he wrote about 90 works. A famous quote from Democritus: "Everything in the universe is the result of chance and necessity."