An adverb meaning 'certainly' or 'without a doubt,' used to emphasize that something is unmistakable.
彼が犯人だと間違いなく言える。
I can say without a doubt that he is the culprit.
この計画は間違いなく成功すると思う。
I think this plan will undoubtedly succeed.
彼女は天才だ。間違いなく。
She's a genius. No question about it.
これは間違いなく君の忘れ物だ。
This is definitely your lost item.