81

Number
$81$ (eighty-one) is:


 * $3^4$


 * The $9$th square number after $1, 4, 9, 16, 25, 36, 49, 64$:
 * $81 = 9 \times 9$


 * The $5$th number, and $1$st square number, whose $\sigma$ value is square: