Not easily excited or flustered; calm and level-headed.
From the phrase "cool head".
- The coolheaded pilot was able to land the plane safely during the emergency.
- She remained coolheaded even when the situation seemed to be out of control.
- He was the most coolheaded person I had ever met.
- The coolheaded detective was able to solve the case.
- The coolheaded negotiator was able to bring the two sides to an agreement.