Subgrade
The natural soil beneath a slab or pavement, after stripping topsoil and organic material. It is the ultimate support for everything above it.
What does subgrade mean?
Uniformity of support matters more than raw strength: a slab bridging between a hard spot and a soft spot cracks even if both areas would be adequate alone. Topsoil, roots, and organics must be removed because they decompose and compress. Soft or expansive subgrades are addressed by undercutting and replacing, stabilizing, or designing the slab to span.
In practice
A slab poured over an unstripped lawn or backfill nobody compacted will crack and settle regardless of its psi.