operator

(noun): One who operates.
(noun): A telecommunications facilitator whose job is to establish temporary network connections.
(noun): A function or other mapping that carries variables defined on a domain into another variable or set of variables in a defined range.
(noun): The game of Chinese whispers.
(noun): A person who is adept at making deals or getting results, especially one who uses questionable methods.
(noun): A member of a military special operations unit.
(noun): The administrator of a channel or network on IRC.
(noun): A symbol that represents a construct in a programming language and differs from a normal function in its syntax.
(noun): A kind of expression that enters into an a-bar movement dependency and is said to bind a variable.

In the sentence "What did Bill say he wants to buy?", "what" is an operator, binding a phonetically empty variable.