a ceramic capacitor
A capacitor is a device that stores electric charge: two conductors held close together but not touching, with an insulator between them. Connect it to a voltage and charge piles up on the two conductors, equal and opposite, held apart by the insulating filling. A ceramic capacitor is simply a capacitor whose insulating filling — the dielectric — is a ceramic, and the classic form is the disc capacitor: a thin ceramic wafer coated with a metal electrode on each face and a wire soldered to each.
How much charge it stores is set by C = epsilon_0 times epsilon_r times A / d, where A is the electrode area, d the ceramic thickness, epsilon_0 the permittivity of free space and epsilon_r the dielectric constant of the ceramic. To pack in more capacitance you want large area, thin dielectric, and above all a high epsilon_r — which is exactly what a ferroelectric ceramic delivers. That single choice of ceramic splits capacitors into two families. Class I dielectrics (paraelectric titanates like titanium dioxide or magnesium titanate) have a modest epsilon_r of tens but are extremely stable and low-loss — the temperature-compensating C0G/NP0 grades used in oscillators and filters where the value must not drift. Class II dielectrics (ferroelectric barium titanate, tailored into X7R, X5R, Y5V grades) have epsilon_r in the thousands, giving far more capacitance per volume, at the cost of temperature, voltage, and age dependence.
Ceramic capacitors are everywhere — smoothing supply rails, coupling signals, decoupling noise, tuning resonant circuits — because ceramics combine high permittivity, good insulation, thermal stability, and cheap high-volume manufacture. The humble disc capacitor was the ancestor; stacking many thin ceramic layers in parallel gave the multilayer ceramic capacitor that now dominates. An honest caveat unique to Class II ceramics: because the dielectric is ferroelectric, its capacitance falls under a DC bias voltage (the DC-bias effect), decays slowly and predictably with time (ferroelectric aging, recovered by heating past the Curie point), and it 'sings' — the converse piezoelectric effect makes the chip vibrate audibly under AC, the notorious capacitor microphonics.
Almost every integrated circuit has a small ceramic 'decoupling' capacitor soldered right beside its power pins. When the chip suddenly draws current, that nearby ceramic capacitor dumps stored charge in a fraction of a nanosecond to hold the voltage steady — a job that depends on the ceramic's high permittivity and very low series resistance.
The whole value of a capacitor rides on its dielectric: a ceramic's high epsilon_r is what lets it store useful charge in a tiny, cheap, robust package.
A high capacitance printed on a Class II ceramic part is its zero-bias, room-temperature value. Apply the rated voltage and warm it up and the real capacitance can be less than half the label — a trap for beginners who take the marking at face value.