A magic square is an arrangement of distinct numbers (i.e. each number is used only once), usually integers, in a square grid, where the numbers in each row, and in each column, and the numbers in the main and secondary diagonals, all add up to the same number. An $n\times n$ magic square that contains the integers from 1 to $n^2$ is called a normal magic square.
Example: Up to rotations and reflections, there is only one $3\times 3$ magic square with entries 1,2,...,9 (which is also known under its Chinese name "Lo Shu"):
   8  1  6