Don't recall the wordage used in Revelation but the second death is permanent separation from God.

The second death is eternal separation from God for all who die without Jesus. How it happens is the Holy Spirit is with all until death. The people with Jesus, the Holy Spirit stays and those without Jesus the Holy Spirit departs from them.

This expression occurs only in the book of Revelation and is clearly symbolic. The Bible gives its own explanation and definition of the symbol by stating: “This means the second death, the lake of fire.” Re 20:14; 21:8. The symbolic quality of the lake of fire is further evident from the context of references to it in the book of Revelation. Death is said to be hurled into this lake of fire. Death obviously cannot be literally burned. Moreover, the Devil, an invisible spirit creature, is thrown into the lake. Being spirit, he cannot be hurt by literal fire. Since the lake of fire represents “the second death” and since Revelation 20:14 says that both “death and Hades” are to be cast into it, it is evident that the lake cannot represent the death man has inherited from Adam (Ro 5:12), nor does it refer to Hades. It must, therefore, be symbolic of another kind of death, one that is without reversal, for the record nowhere speaks of the “lake” as giving up those in it, as do Adamic death and Hades. (Re 20:13) Thus, those not found written in “the book of life,” unrepentant opposers of God’s sovereignty, are hurled into the lake of fire, meaning eternal destruction, or the second death.