Democritus was a Greek philosopher from Abdera who lived during the 5th and 4th centuries BCE, a younger contemporary of Socrates. He is traditionally said to have lived to an old age, with his birth placed around 460 BCE. Known as "the Laughing Philosopher" for his cheerful outlook, he used his wealth to travel widely in search of knowledge. He was a student of Leucippus, the founder of the philosophical system known as atomism.

Democritus was a prolific writer, but his works survive only in fragments quoted by later authors. Ancient sources credit him with over 70 titles covering natural philosophy, ethics, mathematics, and more. His most significant contributions were his writings on atomic theory, which argued that everything in the universe is composed of indivisible particles called atoms moving in empty space, and his ethical works, which consisted of maxims advocating for cheerfulness and practical wisdom.

According to modern scholars, Democritus is a foundational figure for developing and systematizing atomic theory. His materialist explanation of nature challenged the views of other major philosophers and later influenced Epicurean thought. Although his physics was criticized by Aristotle, it was preserved through such engagement. His atomistic ideas were revived in the early modern period, forming a crucial precursor to modern scientific thought.