noun
everyday feelings; usual state of mind; composure
Refers to one's ordinary, calm state of mind, often used in contexts of maintaining composure or returning to a normal mental state. Originates from a Buddhist term.
試合の前は、平常心を保つことが大切だ。
Before a match, it's important to keep your composure.
彼はどんな時でも平常心を失わない。
He never loses his usual calm state of mind, no matter what.
平常 means 'normal, ordinary' in general, whereas 平常心 specifically refers to the ordinary state of mind.
Compound of 平常 (ordinary, usual) and 心 (mind, heart). The term is used in Buddhism to refer to an undisturbed, natural state of mind.