Revelation 17:5

New English Translation

⁵On^[a] her forehead was written a name, a mystery:^[b] “Babylon the Great, the Mother of prostitutes and of the detestable things of the earth.”

Revelation 17:5 tn Here καί (kai) has not been translated because of differences between Greek and English style.
Revelation 17:5 tn Some translations consider the word μυστήριον (mustērion, “mystery”) a part of the name written (“Mystery Babylon the Great,” so KJV, NIV), but the gender of both ὄνομα (onoma, “name”) and μυστήριον are neuter, while the gender of “Babylon” is feminine. This strongly suggests that μυστήριον should be understood as an appositive to ὄνομα (“a name, i.e., a mystery”).

New English Translation (NET)