DBi

From ProPHOTO WIKI

A gain measurement used to quantify the performance of an antenna system relative to a theoretical reference called an isotropic radiator. An isotropic radiator is completely unfocused and emits energy in all directions equally (similar to a bare incandescent bulb). As gain increases, the energy is better focused (like placing a reflector around that bulb) and the range within a specific set of angles can be significantly expanded.

It is important to note that dBi is a logarithmic measure, so every 3dBi results in double the measured output power, every 10dBi results in an order of magnitude increase, etc. This means that these numbers are often deceptively small for the significant improvements that they represent.

Please see antenna gain for more information.