Slab on grade
A concrete floor cast directly on prepared ground, supported by the soil beneath it rather than spanning between supports.
What does slab on grade mean?
Because the soil carries the load, the base preparation matters as much as the concrete: uniform support prevents the differential settlement that cracks slabs. Design decisions include thickness, reinforcement, joint layout, vapor retarder, and whether the edges are thickened or tied to separate footings. Most residential garage floors, basement floors, patios, and driveways are slabs on grade.
In practice
When a slab cracks, the investigation almost always starts under it rather than in it.