The last book of the Bible is called by two names in the English language, but both names have the same meaning.
The Ignatius Catholic Study Bible gives a detailed definition of the last book of the Bible.
The last book of the Bible is then named Apocalypse to closely follow the original Greek word, or Revelation, which is the English equivalent.
Both words have the same meaning and "reveal" the glory of God, both in the present time and in the time to come, specifically the "Wedding Feast of the Lamb" in Heaven.