a substance, e.g. acetylcholine, that is released at a nerve ending and transmits nerve impulses across the synapse (gap between nerve cells)
There are at least 50 different types of neurotransmitter. Nerve endings (synapses) are packed with small vesicles containing neurotransmitters; when an impulse arrives down the nerve it triggers the release of these chemicals, which diffuse across the synaptic junction to the post-synaptic neuron or muscle, where they interact with protein receptors in the membrane, either exciting or inhibiting its activity — Professor Steven Rose
neurotransmission /-ʹmish(ə)n/ noun