1. A hormone secreted by the adrenal medulla that is released into the bloodstream in response to physical or mental stress, as from fear or injury. it initiates many bodily responses, including the stimulation of heart action and an increase in blood pressure, metabolic rate, and blood glucose concentration. also called adrenaline.
2. A white to brownish crystalline compound, C9H13NO3, isolated from the adrenal glands of certain mammals or synthesized and used in medicine as a heart stimulant, vasoconstrictor, and bronchial relaxant.

1. adrenaline: a catecholamine hormone secreted by the adrenal gland in response to stress (when it stimulates the autonomic nervous system).
2. A neurotransmitter which is synthesized from norepinephrine.

1. a catecholamine secreted by the adrenal medulla in response to stress (trade name adrenalin); stimulates autonomic nerve action