Original: In mathematics, the differential coefficient of a function f (x) is what is now called its derivative df (x) / dx, the (not necessarily constant) multiplicative factor or coefficient of the differential dx in the differential df (x).
Simplified: In mathematics, the former differential coefficient of a function f (x) is now called its derivative df (x) / dx. It is the (not necessarily constant) multiplicative factor or coefficient of the differential dx in the differential df (x).