1. [n]  the cardinal number that is the sum of one and one and one
2. [adj]  having three units or components or elements; "a ternary operation"; "a treble row of red beads"; "overcrowding made triple sessions necessary"; "triple time has three beats per measure"; "triplex windows"

1. \Ter"na*ry\, a. [L. ternarius, fr. terni. See {Tern},
   a.]
   1. Proceeding by threes; consisting of three; as, the ternary
      number was anciently esteemed a symbol of perfection, and
      held in great veneration.
   2. (Chem.) Containing, or consisting of, three different
      parts, as elements, atoms, groups, or radicals, which are
      regarded as having different functions or relations in the
      molecule; thus, sodic hydroxide, {NaOH}, is a ternary
      compound.
   

2. \Ter"na*ry\, n.; pl. {Ternaries}.
   A ternion; the number three; three things taken together; a
   triad.
         Some in ternaries, some in pairs, and some single.
                                                  --Holder.
   

A description of an operator taking three arguments. The only common example is C's ?: operator which is used in the form "CONDITION ? EXP1 : EXP2" and returns EXP1 if CONDITION is true else EXP2. Haskell has a similar "if CONDITION then EXP1 else EXP2" operator.

See also unary, binary.