The outer portion
border, frontier, extremity, portal, threshold, brink, boundary, end, term, limit, edging, molding, brim, rim, margin, ring, frame, side, corner, point, bend, crook, hook, split, peak, tip, turn, crust, verge, bound, ledge, skirt, outskirt(s), lip, limb, hem, welt, seam, selvage, terminator, fringe, frill, flange, flounce, list, listing, trimming, mouth, shore, strand, dike, quay, bank, beach, wharf, dock, mole, curb, crest, deckle, berm, groin, arris, gunwale, periphery, circumference, perimeter; see also rim.
Antonyms surface, center*, interior.