Source : Webster's Revised Unabridged Dictionary (1913)

Ternion \Ter"ni*on\, n. [L. ternio, fr. terni three each. See
   {Tern}, a.]
   The number three; three things together; a ternary. --Bp.
   Hall.

Source : WordNet®

ternion
     n : the cardinal number that is the sum of one and one and one
         [syn: {three}, {3}, {III}, {trio}, {threesome}, {tierce},
          {leash}, {troika}, {triad}, {trine}, {trinity}, {ternary},
          {triplet}, {tercet}, {terzetto}, {trey}, {deuce-ace}]