The frost line is the depth where the temperature of soil remains below freezing point. This depth changes from one location to another, and the term is known by various synonyms depending on the region and technical language. In engineering and construction sectors, the frost line is known as the freezing depth or frost depth. It is called the frost point in meteorology, while in architecture, it is known as the frost depth index. Other names for frost line include permafrost depth, ground frost depth, frost penetration depth, and freezing depth. No matter the term, the frost line is a crucial consideration for various applications, including foundation designs, building designs, and farming practices, among others.