C fiber

n.
  1. Any of the unmyelinated fibers, 0.4 to 1.2 micrometers in diameter, conducting nerve impulses at a velocity of 0.7 to 2.3 meters per second.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.