"Gravitational compression" refers to the process of squeezing or shrinking an object due to the force of gravity. When something is under gravitational compression, it is being pressed together by the strong pull of gravity. Full definition
The increase in temperature as you go lower in the atmosphere comes from static gravitational compression of that atmosphere, as predicted by the ideal gas law itself. (notrickszone.com)
Eventually, gravitational compression again will cause the internal temperature until the hydrogen shell around the now inert core begins to fuse into helium, which settles into a middle layer. (solstation.com)
Accounting for gravitational compression, however, Mercury is the second densest after Earth, with 98 percent of its «mean» or compressed density (5.43 versus 5.52 gm / cm3, see NASA Planetary Factsheet). (solstation.com)