broadly, an actor or performer in a masked mime (pantomime), especially the traditional kind of performance that took place during Christmas and other holidays in England during the 18th and 19th centuries;
e.g. Each mummer wore a different colored costume and face fully painted with makeup.
(noun)
a person who wears a disguise such as a mask and attends festivals to go merrymaking;
e.g. You might not be able to distinguish a mummer from the invited guests at a festival.