Resistance in series with an "ideal" capacitor. Sources include lead resistance, terminal losses, etc. An important specification for high frequency applications.

(1) The resistive element, measured in ohms, of a crystal device. The ESR measurement is made only at the series resonant frequency (FS), not at some predetermined parallel resonant frequency (FL). The motional inductance (L1) and motional capacitance (C1) are of equal ohmic value but are exactly opposite in phase. The net result is that they cancel one another and only a resistance remains in the series leg of the above equivalent circuit. Crystal resistance measured at some parallel load resonant frequency is often called the "effective" resistance. (2) A term used to define and quantify the resistive element of a crystal.

Equivalent series resistance (ESR) is an effective resistance that is used to describe the resistive parts of the impedance of certain electrical components.