frostline

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

noun

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

Origin of frostline

An Americanism dating back to 1860–65; frost + line1

British Dictionary definitions for frost line

frost line

noun

the deepest point in the ground to which frost will penetrate
the limit towards the equator beyond which frosts do not occur

Scientific definitions for frost line

frost line

In regions where there is no permafrost, the maximum depth to which frost penetrates the ground in the winter.
The lower limit of permafrost.
In tropical regions, the elevation below which frost never occurs.