The reason for this has to do with finding the area of a square. When you are looking for the area of a square, you use the rectangle formula (since a square is also a rectangle).
The formula is Area = Length * Width
However, since in a square, the length and width are the same, both get changed to the word "Side". As a result, we get the following formula.
Area = Side * Side
This can be simplified to:
Area = Side^2.
Since each number to the second power also shows the area of a square with that given length sides, it can also be called "squared".
