frostline

[ frawst-lahyn, frost- ]
/ ˈfrɔstˌlaɪn, ˈfrɒst- /

noun

the maximum depth at which soil is frozen.
the lower limit of permafrost.

Origin of frostline

An Americanism dating back to 1860–65; frost + line1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019