In mathematics, a **binary operation** or **dyadic operation** is a rule for combining two elements (called operands) to produce another element. More formally, a binary operation is an operation of arity two.

More specifically, an **internal binary operation** *on a set* is a binary operation whose two domains and the codomain are the same set. Examples include the familiar arithmetic operations of addition, subtraction, and multiplication. Other examples are readily found in different areas of mathematics, such as vector addition, matrix multiplication, and conjugation in groups.

An operation of arity two that involves several sets is sometimes also called a *binary operation*. For example, scalar multiplication of vector spaces takes a scalar and a vector to produce a vector, and scalar product takes two vectors to produce a scalar. Such binary operations may be called simply binary functions.

Binary operations are the keystone of most algebraic structures that are studied in algebra, in particular in semigroups, monoids, groups, rings, fields, and vector spaces.

More precisely, a binary operation on a set is a mapping of the elements of the Cartesian product to :^{[1]}^{[2]}^{[3]}

Because the result of performing the operation on a pair of elements of is again an element of , the operation is called a **closed** (or **internal**) binary operation on (or sometimes expressed as having the property of closure).^{[4]}

If is not a function but a partial function, then is called a **partial binary operation**. For instance, division of real numbers is a partial binary operation, because one can't divide by zero: is undefined for every real number . In both model theory and classical universal algebra, binary operations are required to be defined on all elements of . However, partial algebras^{[5]} generalize universal algebras to allow partial operations.

Sometimes, especially in computer science, the term binary operation is used for any binary function.

Typical examples of binary operations are the addition () and multiplication () of numbers and matrices as well as composition of functions on a single set. For instance,

- On the set of real numbers , is a binary operation since the sum of two real numbers is a real number.
- On the set of natural numbers , is a binary operation since the sum of two natural numbers is a natural number. This is a different binary operation than the previous one since the sets are different.
- On the set of matrices with real entries, is a binary operation since the sum of two such matrices is a matrix.
- On the set of matrices with real entries, is a binary operation since the product of two such matrices is a matrix.
- For a given set , let be the set of all functions . Define by for all , the composition of the two functions and in . Then is a binary operation since the composition of the two functions is again a function on the set (that is, a member of ).

Many binary operations of interest in both algebra and formal logic are commutative, satisfying for all elements and in , or associative, satisfying for all , , and in . Many also have identity elements and inverse elements.

The first three examples above are commutative and all of the above examples are associative.

On the set of real numbers , subtraction, that is, , is a binary operation which is not commutative since, in general, . It is also not associative, since, in general, ; for instance, but .

On the set of natural numbers , the binary operation exponentiation, , is not commutative since, (cf. Equation x^{y} = y^{x}), and is also not associative since . For instance, with , , and , , but . By changing the set to the set of integers , this binary operation becomes a partial binary operation since it is now undefined when and is any negative integer. For either set, this operation has a *right identity* (which is ) since for all in the set, which is not an *identity* (two sided identity) since in general.

Division (), a partial binary operation on the set of real or rational numbers, is not commutative or associative. Tetration (), as a binary operation on the natural numbers, is not commutative or associative and has no identity element.

Binary operations are often written using infix notation such as , , or (by juxtaposition with no symbol) rather than by functional notation of the form . Powers are usually also written without operator, but with the second argument as superscript.

Binary operations are sometimes written using prefix or (more frequently) postfix notation, both of which dispense with parentheses. They are also called, respectively, Polish notation and reverse Polish notation.

A binary operation on a set may be viewed as a ternary relation on , that is, the set of triples in for all and in .

An **external binary operation** is a binary function from to . This differs from a *binary operation on a set* in the sense in that need not be ; its elements come from *outside*.

An example of an external binary operation is scalar multiplication in linear algebra. Here is a field and is a vector space over that field.

Some external binary operations may alternatively be viewed as an action of on . This requires the existence of an associative multiplication in , and a compatibility rule of the form , where and (here, both the external operation and the multiplication in are denoted by juxtaposition).

The dot product of two vectors maps to , where is a field and is a vector space over . It depends on authors whether it is considered as a binary operation.