An isotropic antenna
is an antenna
that radiates equally in all directions. There is no actual physical isotropic antenna; a close approximation is a stack of two pairs of crossed dipole antennas driven in quadrature.
Antenna gains are 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.