An isotropic antenna is:

An isotropic antenna is a theoretical concept used in radio physics and antenna engineering. It is defined as a hypothetical point source that radiates energy equally in all possible directions. Such an antenna does not actually exist in the physical world, but it provides a standard mathematical baseline (0 dBi) for measuring the gain of real-world antennas. A dummy load absorbs energy, and a half-wave dipole is a physical antenna with a distinct, non-uniform radiation pattern.