1. A substance, such as norepinephrine, that diffuses into the circulation from the terminals of active sympathetic nerves and serves as a chemical mediator.
2. A neurotransmitter of a substance at sympathetic nerve synapses that is secreted by the sympathetic nerve endings and acts as a neurotransmitter chemical, such as acetylcholine, which is a white crystalline compound that is released from the ends of nerve fibers and is involved in the transmission of nerve impulses.