What is the square root of 256?

##sqrt(256)=16##

at least that's the positive square root

##-16## is also a square root of 256.

##16^2 = (-16)^2 = 256##

By definition, the principal square root of a number is the positive square root. The term "the square root" is commonly used to refer to the principal (positive) root though there are usually two roots.

##sqrt(x^2)=|x|##