Which term describes the rubbing off of skin by friction?

The term that describes the rubbing off of skin by friction is "abrasion." Abrasions occur when the skin is scraped or worn away, often as a result of contact with a rough surface. This process can occur during activities such as sports, falls, or any situation where skin is subject to friction. In terms of injury, abrasions are typically characterized by their superficial damage to the epidermis, which may lead to bleeding or pain depending on the depth of the injury.

Considering the other terms provided: bursitis refers to inflammation of the bursa, which is a small fluid-filled sac that reduces friction between tissues in joints; heat cramps are painful muscle contractions that occur due to loss of electrolytes and dehydration during vigorous exercise, and heat exhaustion is a serious condition resulting from prolonged exposure to high temperatures, characterized by heavy sweating, weakness, and possible fainting due to depletion of water and salts in the body. Each of these terms pertains to different health issues and injuries, making "abrasion" the most accurate choice for describing skin friction injuries.