The Free Dictionary explains that an association neuron is a neuron found in the brain and spinal cord that conducts impulses between neurons. These neurons are commonly referred to as internuncial neurons or interneurons. According to Wikipedia, an interneuron is a neuron that forms connection between other neurons.
Interneurons are neither sensory nor motor neurons, according to Wikipedia. Interneuron is a term reserved for brain and spinal cord neurons whose axons only connect with nearby neurons.
According to Biology-Online, any circuit neuron in the central nervous system that relays impulses between sensory and motor neurons is an interneuron. An interneuron is a multipolar neuron that joins sensory and motor neurons together in a pathway. All nerve cells found in the central nervous system are interneurons. In the human brain, there are approximately 100 billion interneurons.
According to Wikipedia, in the central nervous system, interneuron is used to define small neurons that project impulses locally. Larger neurons project impulses over greater distances across the brain and spinal cord. Central nervous system interneurons that are inhibitory use the neurotransmitter glycine.
An example of an interneuron, as given by Biology-Online, is the Golgi cell, which is found in the cerebellum of the brain.