Definition:Binary Operation

A binary operation is a mapping $$\circ$$ from the Cartesian product of two sets $$S \times T$$ to a universe $$\mathbb{U}$$:

$$\circ: S \times T \to \mathbb{U}: \circ \left ({s, t}\right) = y \in \mathbb{U}$$

If $$S = T$$, then $$\circ$$ can be referred to as a binary operation on $$S$$.

Note that a binary operation is a special case of a general operator, i.e. one that has two operands.

If $$\circ$$ is a binary operation on $$S$$, then for any $$T \subseteq S$$, $$\circ \left ({x, y}\right)$$ is defined for every $$x, y \in T$$. So $$\circ$$ is a binary operation on every $$T \subseteq S$$.

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.

Infix Notation
A far more common alternative to the notation $$\circ \left ({x, y}\right) = z$$ is to put the symbol for the operation between the two operands: $$z = x \circ y$$.

This is called infix notation.