A unipolar neuron is a type of neuron in which only one protoplasmic process (neurite) extends from the cell body. Most neurons are multipolar, generating several dendrites and an axon: unipolar neurons are exceptions to this rule.
Unipolar neurons are common in insects, where the cell body is often located at the periphery of the brain and is electrically inactive. These cell bodies often send a single neurite into the brain; however, this neurite may ramify into a large number of branches making a very complex set of connections with other neurites, in regions of neuropil.
In all species, including vertebrates and invertebrates, many types of primary sensory neurons are unipolar. Typically these have special structures for transducing some type of physical stimulus (light, sound, temperature, etc) into electrical activity, no dendrites, and a single axon that conveys the resulting signals into the spinal cord or brain.