Capacitance is how well something can store charge. Things with capacitance also resist changes in voltage. As we know, charge is simply the collection of electrons. When a voltage is put across a circuit, if there is a capacitance, there will be a current that flows in and fills the capacitance. If the voltage reverses, the charge that has been stored will flow the other way, depleting it. This stored charge and the current flow when the voltage changes slows that change. This makes capacitance extremely useful in many ways but can also cause large problems when its unwanted.
The ability to store electric charge.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz
The ratio of the charge on one plate of a capacitor to the voltage difference between the two plates, measured in farads (F).
Fundamentals of Electric Circuits, 5th Edition by Charles K. Alexander and Matthew N. O. Sadiku
Capacitance is the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors).
The capacitance is a function only of the geometry of the design (e.g. area of the plates and the distance between them) and the permittivity of the dielectric material between the plates of the capacitor. For many dielectric materials, the permittivity and thus the capacitance, is independent of the potential difference between the conductors and the total charge on them.
The SI unit of capacitance is the farad (symbol: F), named after the English physicist Michael Faraday. A 1 farad capacitor, when charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates. The reciprocal of capacitance is called elastance.