Answer
A SYNAPSE is the JUNCTION between the terminal branches of the AXON of one neuron and the DENDRITES or cell body of the next neuron. The two neurons do not actually touch — they are separated by a minute gap called the synaptic cleft.
The essential point, and the one that earns the mark, is that the neurons DO NOT TOUCH. A nervous impulse travels electrically along a neuron, but it cannot jump the gap — so at the synapse it must be converted into a CHEMICAL signal, carried across by a neurotransmitter, and turned back into an electrical impulse in the next neuron.
WHY THIS ARRANGEMENT MATTERS: because transmission across the gap works in only one direction, the synapse ensures that impulses travel ONE WAY through the nervous system. If neurons were joined directly, impulses could run backwards and the system would be chaotic.