operative

Etymology

From Middle French operatif (modern French opératif).

adj

1. Effectual or important.
   He's usually in a good mood — the operative word there being "usually". Today was a disaster.
2. Functional, in working order.
3. Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.
   an operative motive
4. Producing the appropriate or designed effect; efficacious.
   an operative dose, rule, or penalty
5. Based upon, or consisting of, a surgical operation or operations. [from 18th c.]
   operative surgery

noun

1. An employee or other worker with some particular function or skill.
2. A spy, secret agent, or detective.
3. A participant in an operation.