an organic arsenical blister agent; a brown or colorless liquid that is part of the unitary chemical stockpile of vesicants.

Lewisite is a chemical compound from a chemical family called arsines. While it is a colorless and odorless liquid when pure it is usually found as an oily, brown liquid with a distinct odor similar to geraniums. It is a chemical weapon, acting as a vesicant (blister agent) and lung irritant, and can be used in combination with mustard gas as mustard-lewisite.