What Is the Second Death, or Eternal Death?

There is a third type of death in Scripture that is different from spiritual death and physical death. This is known as eternal death, or the second death. In the Book of Revelation we read. 
"The one who has an ear, let them hear what the Spirit says to the churches. The one who overcomes will not be hurt at all by the second death" (Revelation 2:11).
Unless spiritual death is reversed in this life, the result will be eternal death. This is unending separation between God and all who reject Him. 

Believers Not Affected
​Believers are not affected by the second death. 
"Blessed and holy are those who have part in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with Him for a thousand years" (Revelation 20:6).

Lake Of Fire
The final judgment, or the lake of fire, is the time when the second death occurs. 
"Then death and Hades were thrown into the lake of fire. The lake of fire is the second death" (Revelation 20:14).
The second death is reserved for unbelievers. Those who experience this death are all those whose names are not found written in the Book of Life. 
"But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters and all liars - their place will be in the fiery lake of burning sulfur. This is the second death" (Revelation 21:8).

Eternal Death
Eternal death, or the second death, is the ultimate form of separation. If a person dies in a state of spiritual death, they enter eternity separated from God. This is the second death. Once a person has experienced the "second death" there is no hope for them, it is irreversible. 

Summary
Physical death is the separation of the body from the spirit. Spiritual death, the way all of us are born, is the spiritual separation of each of us from God. Eternal death, or the second death, is the eternal separation of a person from God. There is no escape from the second death.