Cool Headed, Calm, Composure
noun, な adjective
You're cool and quiet. That's you, daddio. Nice and cool-headed and calm. So much composure
れいせい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
できるだけ冷静な口調で話すようにした。
I tried to speak in as calm a tone as possible.
落ち着いて！ちょっと冷静になろう。
Let's calm down! Let's compose ourselves a bit.
こんなことが起きて、冷静でいられる訳がないだろ！
How can you stay calm after something like this?