Directivity of an antenna refers to the measure of how concentrated the radiation emitted is in a single direction. It is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. In simpler terms, it is the calculation of radiated power in a particular direction compared to the average radiated power in all directions. This characteristic is crucial for antennas designed to transmit or receive signals in a specific direction, making it an essential parameter for many communication systems.