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Illustration showing f, a function from the pink domain X to the blue codomain Y. The yellow oval inside Y is the image of f. Both the image and the codomain are sometimes called the range of f.

In mathematics, the domain of definition (or simply the domain) of a function is the set of "input" or argument values for which the function is defined. That is, the function provides an "output" or value for each member of the domain.[1] Conversely, the set of values the function takes on as output is termed the image of the function, which is sometimes also referred to as the range of the function.


If the domain of a function is a subset of the real numbers and the function is represented in a Cartesian coordinate system, then the domain is represented on the x-axis.

Graph of the real-valued square root function, f(x) = x, whose domain consists of all nonnegative real numbers


Formal definitionEdit

Given a function  , the set   is the domain of  ; the set   is the codomain of  . In the expression  ,   is the argument and   is the value. One can think of an argument as a member of the domain that is chosen as an "input" to the function, and the value as the "output" when the function is applied to that member of the domain.

The image (sometimes called the range) of   is the set of all values assumed by   for all possible  ; this is the set  . The image of   can be the same set as the codomain or it can be a proper subset of it; it is the whole codomain if and only if   is a surjective function, and otherwise it is smaller.



has no value for  . Thus, the set of all real numbers,  , cannot be its domain. In cases like this, the function is either defined on   or the "gap is plugged" by explicitly defining  . If we extend the definition of   to the piecewise function


then f is defined for all real numbers, and its domain is  .

Any function can be restricted to a subset of its domain. The restriction of   to  , where  , is written  .

Natural domainEdit


Domain of a partial functionEdit


Category theoryEdit


Other usesEdit

The word "domain" is used with other related meanings in some areas of mathematics. In real and complex analysis, a domain is an open connected subset of a real or complex vector space. In the study of partial differential equations, a domain is the open connected subset of the Euclidean space   where a problem is posed, that is, where the unknown function(s) are defined.

More common examplesEdit

As a partial function from the real numbers to the real numbers, the function   has domain  . However, if one defines the square root of a negative number x as the complex number z with positive imaginary part such that z2 = x, the function   has as its domain the entire real line (but now with a larger codomain). The domain of the trigonometric function   is the set of all (real or complex) numbers not of the form  .

See alsoEdit


  1. ^ Paley, Hiram; Weichsel, Paul M. (1966). A First Course in Abstract Algebra. New York: Holt, Rinehart and Winston. p. 16.
  2. ^ Rosenbaum, Robert A.; Johnson, G. Philip (1984). Calculus: basic concepts and applications. Cambridge University Press. p. 60. ISBN 0-521-25012-9.