6

Definition
$6$ (Six) is:


 * The first perfect number:
 * $6 = 1 + 2 + 3 = 1 \times 2 \times 3$


 * The second hexagonal number:
 * $6 = 1 + 2 + 3$


 * The third triangular number:
 * $6 = 1 + 2 + 3$