Equivalent Series Inductance (ESL) means equivalent series inductance. This refers to the inductances that are part of the impedance of electronic components. For example, the inductance of the connecting wires or that of connecting wires within the component.
Generally, when considering components, one assumes that they are ideal and have only the capacitance, inductance, or resistance that is shown. However, all electronic components have lead wires that have undesirable inductance. This means that the component has inductive properties in addition to its intrinsic properties.
In the case of capacitors, this Equivalent Series Inductance becomes disturbingly noticeable at high frequencies. This is due to the fact that the capacitive resistance drops with increasing frequency at 10 dB/ decade, while the inductive resistance increases at 10 dB/decade. To reduce these impairments at higher frequencies, the Equivalent Series Inductance must be minimized.