The act of irritating: the prankster's irritation of the others in the dorm. The condition of being irritated; vexation: honked the horn with irritation at the delay. A source of irritation: Their constant talking is an irritation to people who are reading. An inflammatory reaction of a bodily part. Physiology The elicitation of a response to a stimulus in a plant or animal organ or tissue, especially in a nerve or muscle.