In mathematics, the domain of a function is the set of inputs accepted by the function. It is sometimes denoted by D or Dom, where f is the function. The domain of a function can generally be thought of as "what x can be". More precisely, given a function f, the domain of f is X. In modern mathematical language, the domain is part of the definition of a function rather than a property of it. The domain is represented on the x-axis of the graph of a function in the Cartesian coordinate system.

To find the domain of a function, we need to determine the set of all possible inputs for the function. For example, the domain of f(x)=x² is all real numbers. However, some functions have limits on what is permissible as an input, such as dividing by 0, negative square roots, or negative logs. In such cases, the domain cannot include those values.

The domain of a function is an important concept in mathematics, as it helps us understand the behavior of a function and its range.