Operative describes something that is actively working or in effect right now. When a rule is operative, it's currently being enforced and controlling behavior. When a law becomes operative, it starts actually controlling what people can and cannot do.

In spy novels and movies, an operative is an agent who carries out secret missions. A CIA operative might gather intelligence in a foreign country. These agents are called operatives because they're the ones actively operating in the field, doing the actual work of espionage rather than sitting behind a desk.

The word also means the most important or relevant part of something. If someone says “the operative word is careful,” they're emphasizing that being careful matters most in that situation. When your teacher explains the instructions for a science experiment and says “the operative question is whether the plant gets sunlight,” she's pointing out the key factor that will determine the results.

Notice how operative suggests action and importance: an operative rule is one that's actually controlling behavior, an operative agent is one actively working in the field, and an operative word is the one that really matters.