What is World War 1 mustard powder?

World War I mustard powder refers to the chemical weapon known as mustard gas, which was first used in combat during the war. It is a blistering agent that causes severe skin, eye, and respiratory damage. The term "mustard powder" is derived from its yellowish color and is a misnomer, as it is a toxic gas rather than a powder. Its use marked a significant and controversial development in warfare, leading to widespread suffering and prompting future regulations on chemical weapons.