A mortarboard or cater is an academic cap. The mortarboard consists of a skull cap surmounted by a stiff, square, cloth-covered board and silk tassel. The mortarboard derives its name from its resemblance to the square board used by bricklayers. The mortarboard originated in the old ecclesiastical biretum or barret cap, the ridged sutures of which ... Found on http://www.probertencyclopaedia.com/browse/PM.HTM
a board, usually square, used by masons to hold mortar. · Also calleda cap with a close-fitting crown surmounted by a stiff, flat, square piece from which a tassel hangs, worn as part of academic costume. Found on https://www.infoplease.com/dictionary/mortarboard