Capacitance is a measure of a capacitor's ability to store electric charge. It is defined as the ratio of the electric charge (Q) stored on each conductor to the potential difference (V) between them. The SI unit of capacitance is the farad (F), named after the English physicist Michael Faraday. One farad is defined as the capacitance that stores one coulomb of charge when one volt is applied across it.

In practical applications, capacitors typically have capacitances measured in microfarads (µF), nanofarads (nF), or picofarads (pF), as one farad represents an extremely large capacitance. Capacitors are essential components in electronic circuits, used for filtering, energy storage, coupling, decoupling, and timing applications.

A parallel plate capacitor consists of two conductive plates separated by a dielectric material. The capacitance depends on the plate area, separation distance, and the dielectric constant of the material between the plates. Increasing the plate area or dielectric constant increases capacitance, while increasing the separation distance decreases it.

The dielectric material plays a crucial role in determining capacitance. Different materials have different dielectric constants (relative permittivity), which represent how much better they are at storing electric field energy compared to a vacuum. Common dielectric materials include air, paper, glass, mica, and various ceramics, each offering different trade-offs between capacitance, breakdown voltage, and stability.