Whats the definition of the a square root?

The square root of a number is a value that, when multiplied by itself, gives the number. Example: 4 * 4 = 16, so a square root of 16 is 4. Note that (-4) * (-4) = 16 too, so - 4 is also a square root of 16. The symbol is √ which always means the positive square root.