An isotropic antenna is a theoretical antenna that radiates equally in all directions - horizontally and vertically with the same intensity. The antenna has a gain of 1 dB in the spherical space all around it and has an efficiency of 100%. The concept of an isotropic antenna is often used as a reference antenna for the antenna gain.

Antenna Pattern of an Isotropic Antenna in the Vertical & Horizontal

Many antennas specify gain in dBi (decibels over isotropic), which is the power transmitted by an antenna in specific direction, divided by the power transmitted by an isotropic antenna emitting the same total power.

3-D Antenna Pattern of an Isotropic Antenna