The Oxford Thesaurus An A-Z Dictionary of Synonyms - nub n.

1) projection, protuberance, knob, boss, lump, bump, knop, protrusion, bulge, node, knot; excrescence, swelling, tumescence: Press the small nub on the side to open the door. 2 essence, core, heart, nucleus, crux, point, gist, pith, kernel, nucleus, meat, (sum and) substance, main issue, gravamen: Let's get to the nub of the argument.