Sometimes, individual sacrifices are necessary for the greater good. What's greater good?
The greater good: The benefit of the whole group, rather than just one person.
The scandal involved governmental surveillance of the political opposition. What's surveillance?
Surveillance: The act of watching people closely, especially to make sure they are not doing anything wrong.
We're planning a surprise birthday party for Martha, but you need to keep it under wraps! What's under wraps?
Under wraps: Kept secret.