Fluent (mathematics)
A fluent is a time-varying quantity or variable. The term was used by Isaac Newton in his early calculus to describe his form of a function. The concept was introduced by Newton in 1665 and detailed in his mathematical treatise, Method of Fluxions. Newton described any variable that changed its value as a fluent – for example, the velocity of a ball thrown in the air. The derivative of a fluent is known as a fluxion, the main focus of Newton's calculus. A fluent can be found from its corresponding fluxion through integration.
The terms fluent and fluxion were used in 1878 by W. K. Clifford in Elements of Dynamic: "a quantity must be continuous to be fluent, must therefore be specified either by a line or an angle and rate of change of a length measured on a straight line or circle means velocity of one end of it or difference of velocity of the two ends." He then derives the product rule and quotient rule.