1. The depth in soil below which the ground remains frozen year-round; the boundary between frozen and unfrozen ground.
Usage: Also called 'frost line' or 'permafrost line'; Primarily used in geology, construction, and civil engineering
2. A visible line or mark on a surface caused by frost or ice formation.
Usage: Less common usage; refers to visible patterns or boundaries created by frost

Examples

• Building codes require foundations to be dug below the frostlines to prevent structural damage.
• The frostlines in this region extend approximately three feet deep during winter.
• Engineers must account for frostlines when designing underground utilities.
• The frostlines shift seasonally depending on temperature fluctuations.
• Frost heave occurs when water expands at the frostlines, pushing up soil and pavement.