The verb redact is actually a derivative of the noun redaction rather than the other way round, formed by a linguistic process known as back-formation. Back-formation is a process of word formation in which a shorter word is formed from a longer word that already exists in the language. This occurs by deletion of some kind of imagined affix inspired by words where conventional word formation has taken place. So, for example, edit was derived from editor by analogy with act and actor, even though in the latter case, the verb act existed first, whereas in the former case, the noun editor existed first.