What is a supernatural being supposed by the Irish and Scotch peasantry to warn a family of the speedy death of one of its members, by wailing or singing in a mournful voice under the windows of the house called?

banshee — A supernatural being supposed by the Irish and Scotch peasantry to warn a family of the speedy death of one of its members, by wailing or singing in a mournful voice under the windows of the house. (Webster's 1913.)

Sources