DIFFERENTIAL, adjective An epithet applied to an infinitely small quantity, so small as to be less than any assignable quantity. This is called a differential quantity. The differential method is applied to the doctrine of infinitesimals, or infinitely small quantities, called the arithmetic of fluxions. It consists in descending from whole quantities to their infinitely small differences, and comparing them. Hence it is called the differential calculus, or analysis of infinitesimals.