Albert Barnes Commentary Revelation 20:14

"And death and Hades were cast into the lake of fire. This is the second death, [even] the lake of fire." — Revelation 20:14 (ASV)

And death and hell were cast into the lake of fire. Death and Hades (hell) are here personified, as they are in the previous verse. The declaration is equivalent to the statement in 1 Corinthians 15:26, The last enemy that shall be destroyed is death. (See Barnes on 1 Corinthians 15:26.)

The idea is that death, considered as the separation of soul and body with all its attendant woes, will exist no more. The righteous will live forever, and the wicked will linger on in a state never to be terminated by death. The reign of Death and Hades, as such, will come to an end, and a new order of things will commence where this will be unknown.

There might be that which would be properly called death, but it would not be death in this form; the soul would live forever, but it would not be in that condition represented by the word adhv—hades. There would be death still, but a "second death differs from the first, in the fact that it is not a separation of the soul and body, but a state of continual agony like that which the first death inflicts—like that in intensity, but not in kind."—Professor Stuart.

This is the second death. That is, this whole process described here—the condemnation, and the final death and ruin of those whose names are not found written in the book of life—properly constitutes the second death. This proves that when it is said that death and hell were cast into a lake of fire, it cannot be meant that all punishment will cease forever, and that all will be saved, for the writer goes on to describe what he calls "the second death" as still existing. See Revelation 20:15.

John describes this as the second death, not because it in all respects resembles the first death, but because it has so many points of resemblance that it may be properly called death. Death, in any form, is the penalty of law; it is attended with pain; it cuts off from hope, from friends, from enjoyment; it subjects one who dies to a much-dreaded condition, and in all these respects it was proper to call the final condition of the wicked death—though it would still be true that the soul would live. There is no evidence that John meant to affirm that the second death would imply an extinction of existence. Death never does that; the word does not naturally and properly convey that idea.

The phrase lake of fire is also used in Matthew 25:41.