collocation = an arrangement or juxtaposition of words or other elements, especially those that commonly co-occur, as rancid butter, bosom buddy, or dead serious.
core: atomic core, clean core, Earth’s core, flow core, magnetic core etc.
hard core = the most dedicated, unfailingly loyal faction of a group or organization: the hard core of the separatist movement.
There is still a small hard core of football supporters who cause trouble whenever they can.
layer: the bottom layer, outer layer, top layer, air layer, thin layer, thick layer etc.