The difference between the instantaneous values of two voltages both being biased by a common mode voltage.

a two-wire input that is measured by subtracting the low input from the high input

A differential analog input consists of two input terminals as well as a ground connection. The input measures the difference between the two inputs without (see CMRR) regard to ground potential variations. For example, the two inputs may be labeled Hi and Low, the difference between those two inputs is the measurement returned by the system. The third input, labeled Ground is a reference ground and must be connected to a reference ground at the signal source. A differential input can reject some signal difference between the Low input and the reference ground.