nucleus

axis, base, capital, center, central, core, cynosure, epicenter, eye, focus, ground zero, heart, hub, locus, mecca, navel, nerve center, nexus, omphalos, seat

bottom line, bull's-eye, centerpiece, core, crux, essence, gist, heart, kernel, keynote, meat, meat and potatoes, net, nub, nubbin, pith, pivot, point, root, sum