Question

Define in detail: Relay neuron

Answer 1

these are one of the type of neurons which are neither sensory or motor neurons and these neurons also relay information between other neurons. 

Answer 2

 An interneuron  is a neuron that forms a connection between other neurons. Interneurons are neither motor nor sensory. The term is also applied to brain and spinal cord neurons whose axons connect only with nearby neurons, to distinguish them from "projection" neurons, whose axons project to more distant regions of the brain or spinal cord.Interneurons in the central nervous system