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]