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In mathematics, an expression is called well-defined or unambiguous if its definition assigns it a unique interpretation or value. Otherwise, the expression is said to be not well-defined or ambiguous. A function is well-defined if it gives the same result when the representation of the input is changed without changing the value of the input. For instance if f takes real numbers as input, and if f(0.5) does not equal f(1/2) then f is not well-defined (and thus: not a function). The term well-defined is also used to indicate whether a logical statement is unambiguous.

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Well-defined functions

Let $$A_1, A_2$$ be sets, let $$A = A_1 \bigcup A_2$$ and define $$f: A \rightarrow \{1,2\}$$ as f(a)=1 if $$a \in A_1$$ and f(a)=2 if $$a \in A_2$$ . Then f is well-defined if $$A_1 \bigcap A_2 = \emptyset$$ . If however $$A_1 \bigcap A_2 \neq \emptyset$$ then f is not well-defined because f(a) is ambiguous for $$a \in A_1 \bigcap A_2$$ .

In group theory, the term well-defined is often used when dealing with cosets, where a function f on a quotient group may be defined in terms of a coset representative. Here, a necessary requirement for f to be considered a function is that the output must be independent of which coset representative is chosen. The phrase f is well-defined is used to indicate that this requirement has been verified.

For example, consider $$\mathbb{Z}/2\mathbb{Z}$$ , the integers modulo 2. Since 4 and 6 are congruent modulo 2, a function f whose domain is $$\mathbb{Z}/2\mathbb{Z}$$ must give the same output when the input is represented by 4 that it gives when the input is represented by 6.

A function that is not well-defined is not the same as a function that is undefined. For example, if f(x) = 1/x, then f(0) is undefined, but this has nothing to do with the question of whether f(x) = 1/x is well-defined. It is; 0 is simply not in the domain of the function.
Operations

In particular, the term well-defined is used with respect to (binary) operations on cosets. In this case one can view the operation as a function of two variables and the property of being well-defined is the same as that for a function. For example, addition on the integers modulo some n can be defined naturally in terms of integer addition.

$$[a]\oplus[b] = [a+b]$$

The fact that this is well-defined follows from the fact that we can write any representative of [a] as a+kn, where k is an integer. Therefore,

$$[a+kn]\oplus[b] = [(a+kn)+b] = [(a+b)+kn] = [a+b] = [a]\oplus[b]$$

and similarly for any representative of [b].
Well-defined notation

For real numbers, the product a \times b \times c is unambiguous because (ab)c= a(bc). In this case this notation is said to be well-defined. However, if the operation (here \times) did not have this property, which is known as associativity, then there must be a convention for which two elements to multiply first. Otherwise, the product is not well-defined. The subtraction operation, -, is not associative, for instance. However, the notation a-b-c is well-defined under the convention that the - operation is understood as addition of the opposite, thus a-b-c is the same as a + -b + -c. Division is also non-associative. However, a/b/c does not have an unambiguous conventional interpretation, so this expression is ill-defined.
Other uses of the term

A solution to a partial differential equation is said to be well-defined if it is determined by the boundary conditions in a continuous way as the boundary conditions are changed.

Equivalence relation § Well-definedness under an equivalence relation
Definitionism
Existence
Uniqueness
Uniqueness quantification
Undefined

References
Notes

Weisstein, Eric W. "Well-Defined". From MathWorld--A Wolfram Web Resource. Retrieved 2 January 2013.

Joseph J. Rotman, The Theory of Groups: an Introduction, p. 287 "... a function is "single-valued," or, as we prefer to say ... a function is well defined.", Allyn and Bacon, 1965.

Books

Contemporary Abstract Algebra, Joseph A. Gallian, 6th Edition, Houghlin Mifflin, 2006, ISBN 0-618-51471-6.
Algebra: Chapter 0, Paolo Aluffi, ISBN 978-0821847817. Page 16.
Abstract Algebra, Dummit and Foote, 3rd edition, ISBN 978-0471433347. Page 1.