An isotropic antenna is an ideal antenna that radiates its power uniformly in all directions. There is no actual physical isotropic antenna. However, an isotropic antenna is often used as a reference antenna for the antenna gain.
The antenna gain is often specified in dBi, or decibels over isotropic. This is the power in the strongest direction divided by the power that would be transmitted by an isotropic antenna emitting the same total power.