Equipotential
A surface on which the electric potential is constant. No work is done moving a charge along an equipotential, and the electric field is everywhere perpendicular to it.
Definition
An equipotential surface is a locus of points that all share the same electric potential V. Because moving a charge between two points with the same potential does no net work, moving along an equipotential surface is free. This is why birds sit safely on power lines: the bird's two feet are at the same potential, so no current flows through it.
Equipotentials are always perpendicular to the electric field. The reason is geometric: the electric field points in the direction of steepest potential decrease (E = −∇V), and the gradient is always perpendicular to level surfaces. Field lines and equipotentials form a right-angle grid, like meridians and parallels on a globe. Drawing them together is the standard way physicists visualise electrostatic configurations.
Conductors in electrostatic equilibrium are automatically equipotentials — any potential gradient inside a conductor would drive current until the charges rearranged and flattened the gradient out. This is why every bare metal wire in a circuit is at a single voltage, why the surface of a conducting sphere is an equipotential no matter how charge is loaded onto it, and why the Faraday cage works: its whole body hovers at one potential, screening whatever is inside.