synaptic vesicle

noun

: a small secretory vesicle that contains a neurotransmitter, is found inside an axon near the presynaptic membrane, and releases its contents into the synaptic cleft after fusing with the membrane

When a nerve impulse moves down the axon of a neuron and arrives at an axon terminal, it stimulates synaptic vesicles in the terminal to discharge neurotransmitters.

—Rita L. Atkinson et al., Introduction to Psychology, 9th edition