What part of a neuron relays signals from one neuron to another neuron or to an effector?

Synaptic terminal

Explanation:

Neuron or nerve cell is excitable cell of the nervous system which consists of cell body or soma (karyon), dendrites and an axon. Dendrites are short, numerous filaments responsible for the signal receiving. On the other hand, axon is long "filament" and it originates from the cell body from the swelling called axon hillock. At the end of an axon there are axon terminals (Synaptic terminals) from which neuron can transmit a signal  another cell. Synaptic terminal is axonic end of presynaptic neuron.

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