8

Definition
$8$ (Eight) is:


 * The second cube number after $1$:
 * $8 = 2^3$


 * The sixth Fibonacci number:
 * $1, 1, 2, 3, 5, 8, \ldots$


 * The second octagonal number after $1$:
 * $1, 8, 21, 40, \ldots$

Also see

 * Cube which is One Less than a Square
 * Cubic Fibonacci Numbers