What is antenna gain?

simply gain is a key performance number which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction.

Antenna Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source.

The antenna gain can be determine by ,how the antenna radiates in particular direction or antenna pattern used to know the antenna gain

in electromagnetics it's the combination between directivity and electrical efficiency.

Antenna gain is how big the antenna equipment can increase radiated power and describes how much power is transmitted in the direction of peak radiation to that of an isotropic source.