Democritus Syllabification: (De·moc·ri·tus)Pronunciation: /dəˈmäkritəs, dē-/Definition of Democritus (circa 460-circa 370 bc), Greek philosopher. He developed the atomic theory originated by his teacher Leucippus that explained natural phenomena in terms of the arrangement and rearrangement of atoms moving in a void. Democritus in other Oxford dictionaries Definition of Democritus in the British & World English dictionary