Noun

(en noun)

Someone who does, performs, or executes; an active person, an agent.

* 2002 , , The Great Nation , Penguin 2003, page 295:

Though his name was closely linked to that of Physiocrats, he was less an armchair intellectual like Quesnay or the elder Mirabeau than a doer in the vein of Bertin and Trudaine [...].

* 2008 , Aleksandra Lojek-Magdziarz, The Guardian , 25 Mar 2008:

In schools, submission, not curiosity, was a highly valued virtue. Thinkers were out, doers were in.

Noun

(en noun)

One who operates.

*

, title=(The Celebrity), chapter=1 , passage=The stories did not seem to me to touch life. […] They left me with the impression of a well-delivered stereopticon lecture, with characters about as life-like as the shadows on the screen, and whisking on and off, at the mercy of the operator .}}

A telecommunications facilitator whose job is to establish temporary network connections.

(mathematics) A function or other mapping that carries variables defined on a domain into another variable or set of variables in a defined range.

Chinese whispers.

(informal) A person who is adept at making deals or getting results, especially one who uses questionable methods.

A member of a military Special Operations unit.

(computing) The administrator of a channel or network on IRC.

(linguistics) 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.