Denshi Jisho — Online Japanese dictionary

冷静 れいせい calm; composure; coolness; serenity
Common word, Na-adjective, Noun 
冷静になる れいせいになる to collect oneself; to chill out; to cool off; to recover oneself
Expression, Godan verb with ru ending 
冷静に成る れいせいになる to collect oneself; to chill out; to cool off; to recover oneself
Expression, Godan verb with ru ending 
冷静沈着 れいせいちんちゃく calm, cool, and collected; level and calm
Noun, Na-adjective 

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