The lower altitudinal boundary of a snow-covered area, especially of one that is perennially covered, such as the snowcap of a mountain.
The fluctuating latitudinal boundaries around the polar regions marking the extent of snow cover.
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.