A tetrahedral number, or triangular pyramidal number, is a figurate number that represents a pyramid with a triangular base and three sides, called a tetrahedron. Wikipedia

Polyhedral number representing a tetrahedron Wikidata

A 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. Wiktionary