It is the measure of the ability of a capacitor to store the charge on its plates.
Suppose there is a parallel plate capacitor that is connected across a voltage source V, this will cause positive and negative charges (Q) to be accumulated on the plates.
The charge stored on the plates is directly proportional to the applied voltages, thus Q = CV, where Q = Charge, V = Voltage and C is capacitance and it is proportionality constant.
Thus, C = Q / V
Explanation:
Here capacitance is constant and it does not depend on the voltage applied and the charge of the plates. Because voltage and charge are directly proportional to each other, when voltage is increased charge on plates will increase by the same factor.
So the capacitance of a capacitor will always remain constant until other factors such as - dielectric and distance between plates are changed.