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