mortarboard
noun [ C ] uk/ˈmɔː.tə.bɔːd/ us/ˈmɔːr.t̬ɚ.bɔːrd/
（黑色的）方顶帽，学位帽
a black hat with a square, flat top, worn for formal occasions and ceremonies by some teachers and students at a college or university, and in the past by some schoolteachers