Unary and Binary Function

A Unary Function is called with a single argument.
The domain of a Unary Function is the set of all permissible values for its argument.
The range of a Unary Function is the set of all possible values that it may return.

A Binary Function is called with two arguments.
The domain of a Binary Function is the set of all ordered pairs (x, y) that are permissible values for its arguments.
The range of a Binary Function is the set of all possible value that it may return.

Quoted from http://www.sgi.com/tech/stl