the.com/neurotransmitter

means A chemical messenger released by a neuron that crosses the gap to the next cell and tells it to fire, calm down, or change its behavior.

from Built from neuro- (Greek neuron, 'sinew' or 'nerve') plus transmitter, from Latin transmittere, 'to send across' — trans ('across') and mittere ('to send'). The word itself only became necessary once scientists in the early-to-mid 20th century confirmed that nerve cells talk to each other via chemicals released across a synaptic gap, rather than by direct electrical touch alone — so the term names exactly what it does: a thing that sends something across.