Denis Diderot was a prominent French philosopher, art critic, and writer during the Enlightenment period. He is best known for being the main editor and contributor to the "Encyclopédie," a comprehensive encyclopedia that aimed to compile and disseminate knowledge on various subjects. Diderot was also known for his progressive and radical ideas, advocating for freedom of thought, expression, and education. His works and contributions have had a lasting impact on the fields of philosophy, literature, and art criticism.