Definition:Operation

Informal Definition
An operation (or operator) is an object, identified by a symbol, which can be interpreted as a process which, from a number of objects, creates a new object.

Arity
The number of operands an operator takes is called its arity or its valency.

The following terminology is common:


 * A $0$-ary operation is called a constant operation.
 * A $1$-ary (resp. $2$-ary, resp. $3$-ary) operation is called a unary (resp. binary, resp. ternary) operation.
 * An $n$-ary operation for some natural number $n$ is called finitary.

Comment
It can be seen that, in the same way that a mapping can be seen as a way of "transforming" one element into to another, an operation does the same thing, just with a larger number of operands.

In fact, as we have just defined it, we see that an operation is a generalisation of the concept of the mapping, or (if you like) a mapping is just an operation with only one operand.

There is another way to view an operation. Instead of viewing it as the act of combining two things in a certain way to get a third, we can look upon it as doing something to the first thing with the second to turn it into the third.

Thus, $\circ \left ({a, b}\right)$ can be interpreted as $\circ_b \left ({a}\right)$, where $\circ_b$ is defined as the mapping which performs "$\circ_b$" on a single operand.

For example, take the statement "$1 + 2 = 3$", where the symbol $+$ represents the familiar binary operation of addition of numbers. Thus, we can either view $+$ as being the operation that takes $1$ and $2$ and maps them onto $3$, or we can say that we take $1$, and then we do something to it: we "add $2$", and this turns the $1$ into $3$.

In the case of addition, in a certain sense the first interpretation comes to mind more easily than the second, but if we take the statement "$3 - 2 = 1$", it's more natural to think of this as "doing something" to $3$, that is, to take $2$ off it, to change it into something smaller, that is, $1$.

Both interpretations are equally valid, but depending on the circumstances, one may be more appropriate than the other.