1.NounA number which results from the summation of consecutive triangular numbers, starting from 1. If the triangular numbers in such a summation are depicted as horizontal triangular arrays of units then these arrays can be stacked vertically to yield a tetrahedral array.
"The
n
{\displaystyle n}
tetrahedral number can be calculated with the formula
(
n
+
1
)
3
−
(
n
+
1
)
6
{\displaystyle {(n+1)^{3}-(n+1) \over 6}}
."