in the higher geometry, an infinitely small quantity, or a particle of quantity so small as to be less than any assignable one. It is called a differential, or differential quantity, because frequently considered as the difference of two quantities; and, as such, it is the foundation of differential calculus. Sir Isaac Newton and the English call it a moment, from its being considered as the momentary increase of quantity. See Fluxions.

Differential Equation is an equation involving or containing differential quantities, as the equation $3x^2dx - 2adx + aydx + ady = 0$. Some mathematicians have also applied the term differential equation in another sense, to certain equations defining the nature of facts.