ir·ri·ta·tion 
(ĭr′ĭ-tāshən)
(ĭr′ĭ-tāshən)n.
1.
a. The act of irritating: the prankster's irritation of the others in the dorm.
b. The condition of being irritated; vexation: honked the horn with irritation at the delay.
2. A source of irritation: Their constant talking is an irritation to people who are reading.
3. An inflammatory reaction of a bodily part.
4. Physiology The elicitation of a response to a stimulus in a plant or animal organ or tissue, especially in a nerve or muscle.