An irritant is a substance that causes pain, itching, or discomfort. Chlorine, which is commonly used in swimming pools and hot tubs, is an irritant that dries out skin and reddens eyes.
A physical irritant does just what it sounds like: it irritates your body in some way. For some people, any scent in hand soap or laundry detergent acts as an irritant, causing itchy rashes. A figurative irritant is someone or something that bugs you. The noise your brother makes when he slurps his cereal every morning might, for example, be an irritant. The word is related to irritate, sharing the Latin root irritare, "provoke."