Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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