nature; disposition; temperament; character
quality
《Buddhism》that which does not change according to external influences
「性」 has other readings with different meanings.
彼女は楽観的な性だと言われている。
She's said to be optimistic by nature.
彼の性として嘘がつけないんだよ。
By his nature, he can't lie.
生まれつきの性は変えられないよ。
You can't change your innate nature.