1. An axon. Nerve fibers are identified by intrinsic characteristics (such as diameter, presence of myelin, or neurotransmitter type) or by extrinsic characteristics, such as whether they conduct sensory or motor information (such as a myelinated, large-diameter, cholinergic motor fiber).
Nerve fiber, or simply fiber is more often used in anatomical descriptions, and fiber tract is more commonly used than axon tract. Axon is more commonly used in histological descriptions.

2. An axon or a small group of axons bundled together by glia or Schwann cells.
3. In informal usage, a small peripheral nerve.

A-delta nerve fiber

A lightly myelinated sensory fiber and nociceptor that relays sudden, sharp, fast pain.

adrenergic nerve fiber

A nerve fiber that releases norepinephrine at its synapse when an impulse is transmitted. Most postganglionic fibers of the sympathetic division are adrenergic.

B nerve fiber

A myelinated nerve fiber that transmits autonomic impulses.

C nerve fiber

An unmyelinated nerve fiber that slowly relays sensations of pain from the periphery or viscera to the central nervous system.

cholinergic nerve fiber

A nerve fiber that releases acetylcholine at its synapse when an impulse is transmitted. Sympathetic and parasympathetic preganglionic fibers, parasympathetic postganglionic fibers, and efferent somatic fibers ending in skeletal muscle are cholinergic.

medullated nerve fiber

SEE: Myelinated nerve fiber.

myelinated nerve fiber

A nerve fiber with a myelin sheath.
SYN: SEE: medullated nerve fiber

nonmyelinated nerve fiber

A nerve fiber without a myelin sheath.

postganglionic nerve fiber

A nerve fiber of the autonomic nervous system that terminates in smooth or cardiac muscle or in a gland. Its cell body lies in an autonomic ganglion.

preganglionic nerve fiber

A nerve fiber of the autonomic nervous system that terminates and synapses in one of the autonomic ganglia. Its cell body lies in the brain or spinal cord.