operative

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Being in effect; having force; operating: "Two major tendencies are operative in the American political system” ( Heinz Eulau).
2. adj. Functioning effectively; efficient.
3. adj. Engaged in or concerned with physical or mechanical activity.
4. adj. Of, relating to, or resulting from a surgical operation.
5. adj. Significant; most important; key: The operative word is "low-fat.”
6. n. A skilled worker, especially in industry.
7. n. A secret agent; a spy.
8. n. A private investigator.
9. n. One who works for a political organization, often wielding influence out of public view.

Century Dictionary and Cyclopedia

1. Active in the production of effects or results; acting; exerting force or influence.
2. Efficacious; effective; efficient.
3. Concerned with the actual exercise of power, or the putting forth of effort or labor in the accomplishment of some end; practical.
4. Of, pertaining to, or concerned with operations, as those of surgery.
5. n. A workman; an artisan.
6. n. One who operates in any line of business; an operator: used in certain circumstances as a convenient substitute for detective.

Wiktionary

1. adj. Effectual or important.
2. adj. Functional, in working order.
3. n. An employee or other worker with some particular function or skill.
4. n. A spy, secret agent, or detective.
5. n. A participant of an operation.

GNU Webster's 1913

1. adj. Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.
2. adj. Producing the appropriate or designed effect; efficacious; effective.
3. adj. (Surg.) Based upon, or consisting of, an operation or operations.
4. n. A skilled worker; an artisan; esp., one who operates a machine in a mill or manufactory.
5. n. One who acts as an agent of another, especially a detective or spy.

WordNet 3.0

1. n. someone who can be employed as a detective to collect information
2. adj. effective; producing a desired effect
3. adj. relating to or requiring or amenable to treatment by surgery especially as opposed to medicine
4. n. a person secretly employed in espionage for a government
5. adj. (of e.g. a machine) performing or capable of performing
6. adj. being in force or having or exerting force