n. 25B6; noun
the earth's core : CENTRE, interior, middle, nucleus; recesses, bowels, depths; informal innards; poetic/literary midst.
the core of the argument : HEART, heart of the matter, nucleus, nub, kernel, marrow, meat, essence, quintessence, crux, gist, pith, substance, basis, fundamentals; informal nitty-gritty, brass tacks, nuts and bolts.
25B6; adjective the core issue : CENTRAL, key, basic, fundamental, principal, primary, main, chief, crucial, vital, essential; informal number-one.
peripheral.