Definition of "greathelm"

Robin gripped his shield and his greathelm, and he waited with the other squires on the display ground. Around them the murmur of the crowd continued to swell.

2015, Tim Hall, Shadow of the Wolf, Scholastic Inc.

At first, they would wear a metal cap and mail coif under a larger greathelm, but later helmets such as bascinets had removable visors.

2017, Ken Mondschein, Game of Thrones and the Medieval Art of War, McFarland