What is defined as the space beneath the surface of the earth?

The term that defines the space beneath the surface of the earth is "Subsurface." This term specifically refers to the area below the ground level, which can include minerals, water, and other natural resources found underground. Understanding subsurface rights is crucial in real estate and leasing contexts, as these rights can be separate from surface rights, leading to different ownership and leasing arrangements.

The other terms used in the question have specific meanings that distinguish them from subsurface. Surface rights refer to the rights associated with the land's surface, including the use and enjoyment of the land above ground level. Airspace pertains to the area above the ground, which can also have separate rights in certain real estate transactions. The term "Land" is a broader definition that encompasses both surface and subsurface areas but does not specifically indicate the space underneath the earth's surface. Each of these concepts is fundamental to understanding real property and the various rights that can pertain to different aspects of land ownership and use.