Parameterized post-Newtonian formalism

In physics, the parameterized post-Newtonian formalism or PPN formalism, is a version of the post-Newtonian approximation to theory of general relativity which includes adjustable parameters. It is used as a tool to compare non-linear Einsteinian gravity to simpler Newtonian gravity in the limit in which the gravitational field is weak and generated by objects moving slowly compared to the speed of light.

Source: Wikipedia — Parameterized post-Newtonian formalism (CC BY-SA 4.0)