The term esbat was misunderstood by Margaret Murray and this error was reflected in her famous but discredited academic study on witchcraft. It appears that Gerald Gardner, who had no background in Traditional Witchcraft, was totally unaware of this terminology blunder. So, when Gardner created Wicca he labeled the festivals, "sabbats"; and the Full Moon sabbats, "esbats".
An esbat is more correctly called a Full Moon.
See Full Moon for more information.