5

Definition
$5$ (Five) is:


 * The second pentagonal number:
 * $5 = 1 + 4$


 * The length of the hypotenuse of the smallest Pythagorean triangle: $3 - 4 - 5$


 * The second automorphic number after $1$:
 * $5^2 = 2 \mathbf 5$


 * The smallest prime number of the form $4 n + 1$, and thus, from Fermat's Two Squares Theorem, the sum of two squares uniquely:
 * $5 = 1^2 + 2^2$


 * The smallest prime number of the form $6 n - 1$.