Any of several nerves that arise in pairs from the brain stem and reach the periphery through openings in the skull. There are 12 such pairs in mammals, birds, and reptiles and usually 10 pairs in amphibians and fish.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.