Amplification in the control circuit and therefore the product of the amplification factors (proportional coefficients) associated with the individual elements (controller, controlled subsystem, measuring device) in the open control circuit.
It is a dimensionless variable and does not apply to control circuits with an integrating element (e.g. closed circuit position control system). The loop gain says nothing directly about the accuracy or dynamic performance within this control circuit (see Kv factor (speed/stroke gain)).