As far as I know, the square root (note singular use of ROOT) of a real number refers to the number that multiplies by itself to give the original number and is greater than or equal to 0 (e.g. the square root of 9 is 3). The positive root is taken by convention, unless othwerise specified. To specify the root that is negative, simply write a negative sign in front of the square root, or say the negative square root.
However, the square roots (plural) of a number (over either the complex or real field) are the two numbers that multiply by themselves to give the original number (e.g. the square roots of 9 are 3 and -3, and also the square roots of 2i are 1 + i and -1 - i).
I'm also pretty sure that the square root (singular) of any number over the complex field doesn't properly make sense without further information (e.g. the square root of z, with the positive real part).
However, the square roots (plural) of a number (over either the complex or real field) are the two numbers that multiply by themselves to give the original number (e.g. the square roots of 9 are 3 and -3, and also the square roots of 2i are 1 + i and -1 - i).
I'm also pretty sure that the square root (singular) of any number over the complex field doesn't properly make sense without further information (e.g. the square root of z, with the positive real part).