What is the tiny gap between two neurons called?

The tiny gap between two neurons is called the synaptic cleft.

It lies between the presynaptic neuron, usually at an axon terminal, and the postsynaptic cell. In a chemical synapse, an arriving action potential triggers calcium entry and causes synaptic vesicles to release neurotransmitters into this narrow space. The molecules diffuse across it and bind to receptors on the receiving cell.

The cleft is typically about 20 nanometers wide, or roughly 0.02 micrometers. Its small size allows neurotransmitter concentrations to rise and fall rapidly, helping signals pass efficiently and then stop through reuptake, enzymatic breakdown, or diffusion.

A common mix-up is calling the whole junction a synaptic cleft. The synapse includes the presynaptic terminal, cleft, and postsynaptic membrane; the cleft is only the gap. Electrical synapses are different because gap junctions connect cells directly without a comparable chemical space. The term “synapse” was introduced by Charles Scott Sherrington in 1897.