operative

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

It holds in all operative principles.

Producing the appropriate or designed effect; efficacious.

an operative dose, rule, or penalty

Based upon, or consisting of, a surgical operation or operations.

operative surgery

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

Practically 50 per cent of the female operatives are found in the two groups aged 15 to 19 and 20 to 24 years, while only 18.2 per cent of the female nonoperatives are found in these age groups.

A spy, secret agent, or detective.

A participant in an operation.