Varying definitions of the electrostatic potential arise due to differences in the assumed role of solvent in interfacial electrostatics (e.g., implicit or explicit), or in whether the potential is meant to act on a hypothetical charged species introduced to the system or on an existing species within the system. In this section, we present three common definitions of the electrostatic potential that highlight these differences. In doing so, we will establish a notation that will allow these different potentials to be easily distinguished, compared, and contrasted.