Operator, n.
Etymology: [L.]
1. One who, or that which, operates or produces an effect.
2. (Surg.)
One who performs some act upon the human body by means of the hand, or with instruments.
3. A dealer in stocks or any commodity for speculative purposes; a speculator. [Brokers' Cant]
2 more senses
4. (Math.)
The symbol that expresses the operation to be performed; -- called also facient.
Shades of meaning
Fernald has no entry of its own for operator; he lists it under agent.
He lists it without comment; the entries above explain its neighbors.