The minute gap between presynaptic cell and postsynaptic cell in a chemical synapse, across which the neurotransmitters diffuse into.
This gap acts as a site where neurotransmitters from presynaptic cell (e.g. neuron) are released into by exocytosis and diffuses across to bind with the receptors in the cell membrane of postsynaptic cell. This gap is only a minute space allowing the concentration of neurotransmitters to be raised and lowered rapidly.
Word origin: synaptic > synapse + cleft.
See also: chemical synapse.