An Etymological Dictionary of Astronomy and Astrophysics

A kind of → degenerate matter resulting from hydrogen gas when it is sufficiently compressed to undergo a phase change to liquid or solid state. Metallic hydrogen is thought to be present in compressed
astronomical objects, such as the interiors of the solar system planets Jupiter and Saturn. Above the core of these planets
(at a temperature of 10,000 degrees and a pressure of 3 million bars)
the electrons are squeezed out of the hydrogen atoms and the fluid starts to conduct like a metal.

See also: → metallic; → hydrogen.