10

Definition
$10$ (Ten) is:


 * The second number after $5$ to be the sum of two different squares:
 * $10 = 1^2 + 3^2$


 * The third number after $2$ and $6$ that is not the difference of two squares, as it is of the form $4 n + 2$:
 * $10 = 4 \times 2 + 2$


 * The base of the decimal system

Also see

 * Divisibility by 10
 * Divisibility by Power of 10