Aδ fiber (A delta-fibers) is a type of sensory fiber.
They are thin, myelinated axons with a moderate conduction velocity, or speed of travel of a nerve signal (2 to 30 m/s). These nerve fibers are associated with acute (sharp) pain and therefore constitute the afferent portion of the reflex arc that results in "pulling away" from noxious stimuli (ie: yanking hand away from hot stove). A certain proportion of Aδ fibers are also associated with sensations of temperature (also known as 'cold receptors' in mammals) and pressure. Slowly-conducting, unmyelinated C fibers, by contrast, carry slow, burning pain.
Like other sensory fibers, the Aδ fiber is an extension of a pseudounipolar neuron with its cell body located in a dorsal root ganglion or trigeminal ganglion. Within the spinal cord, afferent nociceptor fibers synapse at or near the spinal cord level where they enter.