Anything that elicits or evokes action in a person or creates a response in a muscle, nerve, gland or other excitable tissue or organ of the body
any substance or agent capable of evoking the activity of a nerve or irritable muscle, or capable of producing an impression upon a sensory organ or more particularly upon its specific end organ
Anything effectively impinging upon any of the sensory apparatuses of a living organism, including physical phenomena both internal and external to the body