The Collaborative International Dictionary of English v.0.48:

Ne plus ultra \Ne plus ul"tra\ [L., no further; ne no, not +
   plus more + ultra beyond.]
   1. The uttermost point to which one can go or attain; hence,
      the summit of achievement; the highest point or degree;
      the acme.
      [Webster 1913 Suppl.]

   2. A prohibition against proceeding further; an insuperable
      obstacle or limiting condition. [Obs. or R.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

ne plus ultra
    n 1: the state of being without a flaw or defect [syn:
         perfection, flawlessness, ne plus ultra] [ant:
         imperfection, imperfectness]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "ne plus ultra":
   acme, acme of perfection, all, apex, apogee, authority,
   authorization, be-all and end-all, blue ribbon, brow, cap,
   capstone, ceiling, championship, climax, cloud nine, command,
   consummation, control, crest, crown, culmen, culmination,
   directorship, dominion, edge, effectiveness, end, extreme,
   extreme limit, extremity, first place, first prize, headship,
   heaven, heavens, hegemony, height, high noon, highest,
   highest degree, highest pitch, highest point, imperium, influence,
   jurisdiction, kingship, last word, leadership, limit, lordship,
   management, mastership, mastery, maximum, meridian, most,
   mountaintop, new high, no place higher, noon, nth degree, palms,
   paramountcy, peak, perfection, pink, pink of perfection, pinnacle,
   pitch, point, pole, power, presidency, primacy, record, ridge,
   rule, say, seventh heaven, sky, sovereignty, spire, summit,
   supremacy, sway, the whole, tip, tip-top, top, top spot, ultimate,
   upmost, upper extremity, uppermost, utmost, utmost extent,
   uttermost, vertex, very top, zenith