What is meant by antenna gain?

Antenna gain is a measure of how effectively an antenna concentrates radio frequency energy in a specific direction compared to a standard reference. This reference is typically either a theoretical isotropic radiator (dBi) or a standard half-wave dipole (dBd). Unlike an electronic amplifier, an antenna does not magically create more power. It simply reshapes and focuses the existing transmitter power, similar to how a flashlight reflector focuses a light bulb's omnidirectional light into a tight beam. Distractors like the ratio of forward-to-back signal actually describe the 'front-to-back ratio', not overall gain. Comparing radiated power directly to transmitter power ignores the directional focusing aspect, which is the true defining characteristic of antenna gain.