inner core

Short definition from Wiktionary

Noun

1. 1
   The solid material found in the centre of some planets at extremely high temperature and pressure, distinct from the liquid outer core.
2. 2
   The solid material found in the centre of some planets at extremely high temperature and pressure, distinct from the liquid outer core: The innermost part of the Earth, believed to be made of a nickel-iron alloy.
3. 3
   Used other than figuratively or idiomatically: see inner, core.