Hi, it suddenly struck me as to what is meant by the square root of a given function $f$. This should be a precalculus question, but I realize I never paid attention to what it really meant. It occurs in the GRE Math Subject sample test.

So, given a function $f$ on the real line, what does $\sqrt{f}$ mean?

For instance, if $f(x)=(-x)^2=x^2$, does $\sqrt{f}=-x$ or $x$?

*I couldn't find a satisfactory answer on the internet and looking up textbooks didn't seem to yield helpful result either.