In mathematics, the term "domain" is often used to refer to the set of input values of a function, while the term "range" refers to the set of output values.
For example, consider the function f(x) = x^2. The domain of this function is all real numbers, because we can input any real number into the function. The range of this function, however, is all non-negative real numbers, because the function only produces positive or zero outputs.
In the context of two-dimensional coordinate systems, such as the Cartesian plane, the terms "x" and "y" typically refer to the horizontal and vertical axes, respectively. The domain and range of a relation or function in this context are often defined in terms of the values of x and y.
For example, the unit circle in the Cartesian plane can be described as the set of all points (x, y) such that x^2 + y^2 = 1. In this case, the domain and range of the relation are both the set of real numbers between -1 and 1, since these are the possible values of x and y that satisfy the equation.
So, to answer your question, whether "domain" refers to x or y depends on the context in which it is being used. In the context of functions, the domain typically refers to the set of input values, while in the context of coordinate systems, x and y are often used to refer to the horizontal and vertical axes, respectively.
