The expression dBi is used to define the gain of an antenna system relative to an isotropic radiator. The symbol is an abbreviation for "decibels relative to isotropic."
The dBi specification is based on the decibel, a logarithmic measure of relative power. Suppose an antenna X produces an electromagnetic field of intensity Ix microwatts per square meter (IxµW/m2) in its favoured direction at a point located some distance away. Also, suppose an isotropic antenna Q produces an electromagnetic field of intensity IQ µW/m2 at the same distance. Then the gain G of antenna X, in dBi, is:
G = 10 log10 (Ix / IQ)
A dipole antenna has a gain of 2.15 dBi. An isotropic radiator or "point source" has a gain of 0 dBi.