When using the phrase "square root", one normally refers to the positive,
real root.
But, any complex number has n nth roots, thus two square roots. Take a real number
x = x + 0*i = x * e0*i = x * e2*pi*i
The square roots are
x1/2 * e1/2*0*i = x1/2 * 1 = x1/2
and
x1/2 * e1/2*2*pi*i = x1/2 * -1 = -x1/2
Update 2002-11-12: The definition of a square root has disappeared! I suggest the following, simple definition:
The sqare root of a number x is a number whose square is x.
I am sure someone can node a much more mathematical definition than this.