sub·grade \ˈsəb+ˌ-\noun Etymology:sub- + grade 1.: a subordinate level in a scale (as of rank or quality) < subgrade is used in some classifications at a supraphylar level > 2.: a layer (as a stratum or surface) immediately beneath some principal layer; specifically: a surface of earth or rock leveled off to receive the foundation of a structure (as a road, pavement, building, sewer) or the ballast of a railroad