Charles Buck Theological Dictionary 
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
a class of monks who are said to have performed divine service without intermission, by dividing themselves into three classes, and taking their turns at the service at stated hours. (See Acoemetae). The term is applied to the keepers of the Easter sepulchre. Usually there were two or three who sang psalms and maintained the watch. The term is also used to designate the keepers of the Church who went the rounds at night.