Fetching details from the dictionary...

inner thoughts; real intention; inmost heart; one's mind
at heart; deep down; on the inside
inner center (centre)
内心では反対だったが表向きは賛成するしかなかった。
I was opposed inwardly, but had no choice but to agree outwardly.
彼女は内心穏やかではなかったが冷静を装っていた。
She was not calm inwardly but pretended to be composed.
内心の葛藤を誰にも打ち明けることができずにいる。
I can't confide my inner conflict to anyone.