triangular numbers

plural noun

: the successive sums n(n + 1)/2 where n = 1, 2, 3, … of the first n natural numbers 1, 3, 6, 10, 15, … representable by dots arranged in triangles

Word History

First Known Use

1795, in the meaning defined above

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

• More than 250,000 words that aren't in our free dictionary
• Expanded definitions, etymologies, and usage notes
• Advanced search features
• Ad free!