The term "subgrade" refers to the layer of soil or material that lies beneath a road, pavement, or foundation. It serves as the foundation for these structures, providing support and stability. The quality and characteristics of the subgrade are crucial, as they affect the overall performance and durability of the surface built on top. Proper preparation of the subgrade involves compaction and grading to ensure it can bear the weight of the structure. Engineers often assess the subgrade's strength and drainage properties to prevent issues like settling or cracking in the pavement or foundation above.