Equivalent Series Resistance (ESR)
It is the internal resistance appearing in series with a capacitor's ideal capacitance, given in the milliohm range. The ripple current produces heat of P = I² × ESR across this resistance; 5 A rms ripple and 50 mΩ ESR means 1.25 W of heat inside the capacitor.
ESR matters for two reasons. First, it limits the capacitor's ability to suppress voltage ripple: the current pulse creates an instantaneous voltage drop across the ESR, and this drop appears directly on the supply bus. Second, the heat is generated inside the capacitor, that is, in the body of the component most sensitive to temperature.
In electrolytic capacitors this heat causes the electrolyte to dry out over time; the capacitance falls, the ESR rises and the heat grows further. It is commonly accepted that every 10 °C increase in operating temperature roughly halves the life of an electrolytic capacitor.
The field counterpart is this: a driver that worked without trouble for years becoming unstable over time may be related to it sitting in a hot environment and to the ageing of the capacitor at its input. For this reason avoid mounting the driver directly on the motor body, on an exhaust line or inside a closed box exposed to the sun; a board working inside a ventilated enclosure and in a vertical position lasts noticeably longer.
The context in which this term is used: DC Motor Driver Selection Guide