Definition:Operation/Arity

The arity of an operator is the number of operands it uses.

An alternative (older) term is valency.

The arity of an operator may be, in general, any number.

A unary operator (or one-place operator) is an operator which takes one operand, that is, its arity is 1.

An example of a unary operator from arithmetic is "$$\sqrt{}$$" (i.e. the square root sign).

A binary operator (or two-place operator) is an operator which takes two operands, that is, its arity is 2.

Certain types of operator have a fixed number of operands. An operator with $$n$$ operands is sometimes called an "$$n$$-ary operator".

Certain other types of operator have a variable number of operands. An operator which does not have a fixed arity is called multiary.

It is possible to conceive of an operator which takes no operands.

A constant can be considered as an operator which takes no operands, and there are circumstances in mathematics and logic in which it is advantageous to do so.