The Collaborative International Dictionary of English v.0.48:

Triangular \Tri*an"gu*lar\, a. [L. triangularis: cf. F.
   triangulaire.]
   1. Having three angles; having the form of a triangle.
      [1913 Webster]

   2. (Bot.) Oblong or elongated, and having three lateral
      angles; as, a triangular seed, leaf, or stem.
      [1913 Webster]

   Triangular compasses, compasses with three legs for taking
      off the angular points of a triangle, or any three points
      at the same time.

   Triangular crab (Zool.), any maioid crab; -- so called
      because the carapace is usually triangular.

   Triangular numbers (Math.), the series of numbers formed by
      the successive sums of the terms of an arithmetical
      progression, of which the first term and the common
      difference are 1. See Figurate numbers, under
      Figurate.
      [1913 Webster]