noun

1. The act of irritating or annoying

   “What irritation causes you to be so moody?”

2. The state of being irritated

3. The act of exciting, or the condition of being excited to action, by stimulation; -- as, the condition of an organ of sense, when its nerve is affected by some external body; especially, the act of exciting muscle fibers to contraction, by artificial stimulation; as, the irritation of a motor nerve by electricity; also, the condition of a muscle and nerve, under such stimulation.