Final answer:
The primary characteristic of a third degree burn is charring. These burns extend fully into the epidermis and dermis, destroying tissue and affecting nerves, and require professional medical attention for proper healing.
Step-by-step explanation:
The primary characteristic of a third degree burn is charring, which signifies that the burn has fully extended into the epidermis and dermis, destroying the tissue and affecting the nerve endings and sensory function. Unlike first-degree burns that present with redness and might heal on their own, or second-degree burns featuring inflammation and blisters, third-degree burns are more severe and may appear white, red, or black, and always require medical attention to heal. These burns can be quite severe and while they might actually be less painful due to nerve damage, they still represent a serious medical condition that demands immediate care.