what's in one's heart; one's emotional state; what one feels
Refers to one's inner thoughts, feelings, or true emotions, often contrasted with outward appearance.
He rarely reveals what's in his heart.
Behind her smile, I sensed complex feelings.
心情 is a more formal, abstract term for one's emotional state or sentiment. 胸の内 is more concrete and personal, literally 'inside one's chest'.
Standard kanji spelling.
Kana-mixed spelling, common in everyday writing.
Rare kanji form using 裡; it is rarely used.
Compound of 胸 (chest, heart) + の (possessive particle) + 内 (inside). Literally 'the inside of one's chest', used figuratively for one's inner thoughts and feelings.