frontal sinus

n.
  1. A hollow formed on either side in the lower part of the squama of the frontal bone, communicating by the ethmoidal infundibulum with the middle meatus of the nasal cavity of the same side.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.