The Collaborative International Dictionary
Triangular \Tri*an"gu*lar\, a. [L. triangularis: cf. F. triangulaire.]
Having three angles; having the form of a triangle.
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(Bot.) Oblong or elongated, and having three lateral angles; as, a triangular seed, leaf, or stem.
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 (Zo["o]l.), 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.
Wiktionary
n. (plural of triangular number English)
Usage examples of "triangular numbers".
Now, all the numbers in the second row are triangular numbers, which means that these numbers of cannon balls may be laid out on the ground so as to form equilateral triangles.
Thus too, he played games with triangular numbers, showing the way they built up one on top of the other.