Final answer:
The domain in a linear function relates to the x-variable which is the independent variable usually represented along the horizontal axis on a graph.
Step-by-step explanation:
In a linear function, the domain typically represents the set of all possible values that the independent variable can take. In the standard form of the linear equation y = a + bx, x is the independent variable and y is the dependent variable. Considering the function's representation on a graph, x-variable values are plotted along the horizontal axis, which corresponds to the domain of the function. Therefore, when referring to a linear function, the domain corresponds to the x-variable.