An employee or other worker with some particular function or skill.

A spy, secret agent, or detective.

A participant in an operation.

Effectual or important.

“He's usually in a good mood — the operative word there being "usually". Today was a disaster.”

Functional, in working order.

Having the power of acting; hence, exerting force, physical or moral; active in the production of effects.

“an operative motive”