Death - means a “separation”

Eternal Death

Wages of sin is death:

Romans 6:23 

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.

Sinners are worthy of death (deserve to die):

Romans 1:32 

Although they know God's righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.

Lust leads to eternal death:

James 1:14-15 

14 but each one is tempted when, by his own evil desire, he is dragged away and enticed. 
15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.

This is the second death, the lake of fire:

Revelation 20:14-15

14 Then death and Hades were thrown into the lake of fire. The lake of fire is the second death. 

15 If anyone's name was not found written in the book of life, he was thrown into the lake of fire.

Revelation 21:8 

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.”

It is eternal punishment:

Matthew 25:41 

“Then he will say to those on his left, ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.

Matthew 25:46 

“Then they will go away to eternal punishment, but the righteous to eternal life.”