An arm-worn shield used in Europe from the 12th to the 18th centuries, though it had it's greatest popularity in the early-to-mid Middle Ages. It is regarded as the "classic" medieval shield and the shield most commonly thought of in conjunction with soldiers of that time in that part of the world.
The basic design of the heater is a shield is either banded wood, in the earlier days, or steel in the latter. It would properly be the width of the soldiers body at the shoulders and the height would be usually be near the length of the shoulders to the waist.
If metal, a slight lateral curve was common to allow better deflection of blows. on the reverse side there would be a rigid handle to be grasped by the hand and a leather or metal loop which the forearm would be through. A proper traditional heater would have the top upper corners slightly raised in peaks, though a flat top was also common. However the most distinctive point of the heater is the tapered bottom, where the top upper corners taper inward as the decent until they both meet in a point at the bottom center of the shield.
The heater design is a direct descendant of the Norman Kite Shield (which is itself from the Door Shield), and there exists also a Norman "Long Heater", where the height of the heater is is extended to go from the shoulders to below the knee.
Virtually all coats of arms and heraldic emblems of any antiquity with a shield on them use the heater design.