gnomon

1. (geometry) A plane figure formed by removing a parallelogram from a corner of a larger parallelogram.Examples: "If a Parallelogram be divided into four leſſer ones, by two Lines interſecting each other; and one of theſe Parallelograms be retrench'd, or taken away; the other three will make a Gnomon, ordinarily call'd a Square."

2. (mathematics, by extension) A number representing the increment between two figurate numbers (“numbers equal to the numbers of dots in geometric figures formed of dots”).Examples: "[page 16] The figurate number's gnomon is therefore 1#43;nb in the general case. A trivial family of numbers, those for which b#61;0, might be called linear. […] [page 17] Clearly, a figurate number of rank n is equal to the sum of its first n gnomons, and in particular the sum of the first n odd numbers is equal to n²."broadly