operative

Short definition from Wiktionary

Adjective

1. 1
   Effectual or important.
2. 2
   Functional, in working order.
3. 3
   Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.
4. 4
   Producing the appropriate or designed effect; efficacious.
5. 5
   Based upon, or consisting of, a surgical operation or operations.

Noun

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