A molecule must meet these very specific criteria to be considered a neurotransmitter: 

• The molecule must be synthesized inside a neuron and must be located in synaptic vesicles that are located in the axon terminal of the neuron.
• The molecule must be released as a result of an action potential
• On arrival of the action potential, the molecule must be released into the synaptic gap between cells
• The molecule must produce appropriate presynaptic and postsynaptic responses
• After it has finished transmitting its signal, the molecule must be rapidly deactivated or eliminated from the synaptic cleft

More than 60 different molecules that are currently known to meet these criteria are considered neurotransmitters.