In algebra, the "domain" of a relation is the set of unique and acceptable INPUT values to the function. In typing out a domain, you'd eliminate any duplicates:
{1, 2, 2, 3, 4, 5, 7} (wrong, because 2 shows up twice)
and you'd ensure that the function/relation is defined at each INPUT value.
Note that the function y = sqrt(x) has domain [0, infinity), so negative numbers are not part of the domain; they are not acceptable input values.