Used to express strong conviction that something is definitely the case; often appears at the end of a statement or as a standalone interjection. Similar to 間違いない, but 間違いなし is slightly more casual and emphatic.
あの店のラーメンは美味い。間違いなし！
That shop's ramen is delicious. No doubt about it!
この計画なら成功は間違いなしだ。
With this plan, success is certain.
間違いなく is the adverbial form meaning 'without fail' or 'undoubtedly', used to modify verbs or adjectives. 間違いなし is a predicative expression that asserts certainty about a situation.
Standard kanji spelling; the kana なし is more common than 無し in modern usage.
Alternative spelling using the kanji 無 for なし; somewhat more formal or emphatic.
Compound of 間違い (mistake, error) + なし (classical terminal form of ない, 'not exist'). Literally 'there is no mistake', used idiomatically to mean 'certain' or 'no doubt'.