An esbat is a Wiccan meeting held each month at the time of the full moon. This is usually the time for initiation ceremonies or healing magic to be done, as opposed to a Sabbat (holiday) celebration.
There's also an old joke in the Wiccan community about "what do you feed your esbat?"
The answer? Purina Esbat Chow!
Examples: For this month's Esbat, Three Circles Coven is planning a healing ceremony.