No, not that one....
In normal parlance, the frost line is the soil depth at which the moisture in soil typically freezes during winter. Below a certain depth, the soil never freezes, and of course it gets really warm as you continue to go deeper.
As you might expect, the frost line is 0 inches in temperate regions, and in such areas it tends to refer to the altitude at which you might have ground frost. As you move away from the equator, the ground starts to freeze in winter. The frost line then refers to how deeply the freeze reaches underground. This value increases as you move away from the ocean, and as you move up towards the poles. In the southern U.S. and lower west coast, this value may be 4 to 5 inches or less. In the mid-west and central east, it's as much as two feet. As you approach the Canadian border it's at least three to four feet, and beyond that, well, it's pretty deep. Eventually you begin to encounter permafrost where the topmost layer of soil never thaws.
It's important to know where the frost line is so that you can bury water mains, storm drains, and sewer lines below that. No one wants those to freeze! (Sewer lines need to be deeper than basements, too, because otherwise things become...sticky. Ten feet below grade is usually a good depth for sewers.)
Support posts for decks and fences need to be deeper than the frost line, too. Frozen ground swells and heaves and a fence or deck post not seated below the frost line won't stay vertical for long. Deck building contractors are notorious for trying to get away with shallow post holes, so prospective deck owners need to be vigilant.
Wikipedia tells me that the astrophysics also has a frost line, which has a different meaning. Take care if you're building a deck on Ceres to be sure you're using the term correctly.
IN9