noun Date: 1795 a number (as 3, 6, 10, 15) representable by that many dots arranged in rows that form a triangle and that equals n(n + 1)/2 for some positive integer value of n
TRIANGULAR NUMBER
Meaning of TRIANGULAR NUMBER in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012