a board, usually square, used by masons to hold mortar.
2.
Also called cap. a cap with a close-fitting crown surmounted by a stiff, flat, square piece from which a tassel hangs, worn as part of academic costume.
Tapper not only made appearances at civic events, he donned a mortarboard and became an online tutor for local schoolchildren.
British Dictionary definitions for mortarboard
mortarboard
/ˈmɔːtəˌbɔːd/
noun
1.
a black tasselled academic cap with a flat square top covered with cloth
2.
Also called hawk. a small square board with a handle on the underside for carrying mortar
Word Origin and History for mortarboard
n.
also mortar-board, "academic cap," 1854, probably from mortar (1) + board (n.1); so called because it resembles a mason's square board for carrying mortar. Earlier it was called a mortar cap (1680s) or simply morter (c.1600), from French mortier.