Taking exactly one argument
, as compared to a binary operator
Addition (+) is a binary operator -- it needs two things, one on either side.
Factorial (!) is a unary operator -- it needs one thing, on its left side.
Increment (++) is a unary operator -- it needs one thing, and behaves differently depending on whether its operand is on the left or the right.
The conditional operator ?: is a ternary operator (Larry Wall calls it a trinary operator in the Camel Book) -- it needs three things and returns the second or third depending on whether the first is true or false.