The "E-plane" refers to a specific plane in the context of electromagnetic waves, particularly in antenna theory. It is defined as the plane that contains the electric field vector of the wave. In this plane, the electric field oscillates, while the magnetic field is perpendicular to it, which is essential for understanding how antennas radiate and receive signals. In practical applications, the E-plane is crucial for designing antennas like dipole antennas and microstrip antennas. By analyzing the E-plane radiation pattern, engineers can optimize antenna performance, ensuring effective communication in various technologies, including cellular networks and Wi-Fi.