My definition of the square root is as follows: the square root of a positive real number x is the positive number, noted √x, such that (√x) ^ 2 = x. To make this a useable definition, we need to prove that equation has a solution (using the fact the function t -> t^2 is zero for t=0, diverges to +inf when t -> +inf, and is continuous between the two) and that solution is unique (using the fact the same function is strictly increasing).
Do you have any other definition of √x?
x^2 = a
where a is a positive real number has two solutions. An equation can have multiple solutions (even infinitely many solutions). Sqrt(x) is a function and as such it can have only one output for a given input. Functions don’t have solutions. By definition, sqrt(a) is the nonnegative solution of the equation x^2=a. Here, a can be zero.