A synapse is the means by which a neuron is able to pass a chemical or electrical signal from one cell to another. The word synapse comes from Greek words meaning 'to clasp together'.

Neurons are cells that pass 'messages' to certain other cells, and the synapse is the structure that lets them do it. At a synapse, the neuron that is passing the signal has a plasma membrane that comes into contact with the membrane of the cell that will receive the signal. There are two main types of synapse:

In an electrical synapse, cell membranes are connected by channels that can pass electric currents, allowing fast transfer of signals between cells.

A chemical synapse involves the release of a chemical called a neurotransmitter,
which may create an electrical response or a different messenger pathway.

You can see a diagram of this process here.