16

Definition
$16$ (sixteen) is:


 * The $4$th square number after $1, 3, 9$:
 * $16 = 4 \times 4$


 * The $2$nd integer power after $1$:
 * $16 = 2 \times 2 \times 2 \times 2$


 * The first square number to be the sum of two triangular numbers in two different ways:
 * $16 = 10 + 6 = 15 + 1$