What does the term “ground gain” mean?

In antenna theory, "ground gain" refers to the effective increase in a transmitted signal's strength at a distant receiver caused by the reflection of radio waves off the earth's surface. When the direct wave from the antenna and the wave reflected from the ground arrive in phase at a distant point, they combine constructively. This constructive interference can increase the effective radiated power in certain directions by up to 6 dB. Distractors incorrectly associate this term with physically connecting the antenna to an electrical ground or altering the antenna's net gain by physically burying part of it.