Equivalent Series Resistance,  dc resistance of the inductor or leakage Resistance in a cap.

The theoretical treatment of devices such as capacitors and inductors tends to assume they are ideal or "perfect" devices, contributing only capacitance or inductance to the circuit. However, all physical devices are constructed of materials with finite electrical resistance, which means that physical components contain some resistance in addition to their other properties.

Q factor is quoted in inductor data sheets, which is a more convenient way (than analyzing ESR) to show the typical high-frequency deviations from the ideal in the inductor magnetics and losses.