1. extremely secret; kept highly confidential

Examples

• The government’s ultrasecret military project was known only to a few officials.
• She worked on ultrasecret research that could not be discussed outside the lab.
• The company’s ultrasecret formula was locked in a vault.
• Access to the ultrasecret files required the highest security clearance.
• The ultrasecret meeting was held at an undisclosed location.
• Only three people knew about the ultrasecret operation.