An IsotropicAntenna is one, which radiates equally in all directions. This is a theoretical construct used mainly to describe the performance of a real antenna in a particular direction. If an antenna radiates more in one direction than another, it is said to have gain in that preferred direction.
Antenna manufacturers usually describe the gain of their antennas in a particular direction with respect either to an isotropic antenna (in dBi) or to a half-wave dipole antenna (in dBd). Further, the gain value is usually expressed in DeciBels (dB), which is a logarithmic quantity, rather than linear.