Privilege

Privilege isn't a casual concept. It's a court-recognized rule that says some categories of information are too important to society to be forced into evidence. The protection has limits: you can waive it (by sharing the information with someone outside the privileged relationship), and there are exceptions (like the crime-fraud exception that strips attorney-client privilege when the lawyer is being used to commit a crime).

If you're claiming privilege to refuse to answer a discovery question, you have to log what you're withholding (a "privilege log") so the other side can challenge whether the privilege actually applies.