**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]