oblong number

An integer of the form $n\cdot (n+1)$ or $n^2+n$. The partial sum

$$\sum _{i=0}^{n}2i$$

works, too. From this it follows that one can also multiply the $n$th triangular number. Plenty of other relations to various polygonal numbers can be obtained.

The first few terms are 0, 2, 6, 12, 20, 30, 42, 56, 72, 90, 110, and are listed in A002378 of Sloane’s OEIS.