1. 1. An international border.
   2. The area along an international border.
2. A region just beyond or at the edge of a settled area.
3. An undeveloped area or field for discovery or research: theories on the frontier of astrophysics.

[Middle English frountier, from Old French frontier, from front, forehead, front. See front.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.