The main, broader question is whether a solution set of a system of equations can ever be such that it can't be described by a single equation.
To be more precise, suppose I have a system of $m$ equations in $n$ variables
$f_1(x_1,x_2,...,x_n)=0, f_2(x_1,x_2,...,x_n)=0, ... f_m(x_1,x_2,...,x_n)=0$
(The $f_i$s are assumed to be polynomials for the purpose of this post).
If one restricts the variables and the values of the $f$s to the real number the answer to the above question is no.
The equation $f_1(x_1,x_2,...,x_n)^2+f_2(x_1,x_2,...,x_n)^2+...+f_m(x_1,x_2,...,x_n)^2=0$ has the same solution set as the system of equations.
How about if one restricts the values to complex numbers? Is there any known context where systems of equations are more expressible than single equations in the above sense?