C_m, or the capacitance of a capacitor, is a measure of its ability to store electrical charge. It is defined as the amount of charge stored per unit voltage across the capacitor's plates. The unit of capacitance is the farad (F), which indicates how much charge a capacitor can hold at a given voltage. Capacitance depends on several factors, including the surface area of the plates, the distance between them, and the type of dielectric material used. Common dielectrics include materials like ceramics, plastic, and air. Understanding C_m is essential in designing circuits and electronic devices, as it affects how they store and release energy.