The square of a number $x$ is the number obtained multiplying $x$ by itself. It's denoted as $x^2$

Some examples: \begin{eqnarray*} 5^2&=&25\\ \left(\frac{1}{3}\right)^2&=&\frac{1}{9}\\ 0^2&=&0\\ (-0.5)^2 &=& 0.25 \end{eqnarray*} A perfect square is the square of an integer. The first few perfect squares are 0, 1, 4, 9, 16, 25, $\ldots$