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A square is a special rectangle, one in which all sides are the same length. Because a square is a kind of rectangle, it must have a right angle at each corner. So, all of the angles are "the same" (congruent, or matching), and all of the sides are congruent, too.
When you multiply an integer times itself, the resulting product is called a perfect square or a square number, or just "a square." So 1, 4, 9, 16, 25, 36, 49, ... 100, 121, 144, ... 9801, ... are all square numbers.