The second death is mentioned several times in the book of Revelation and it refers to the lake of fire.  It is called the second death because it follows physical death.

Revelation 2:11 states: “He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death.”  So the “overcomer” does not have to fear the second death.  Who is an “overcomer”? – “For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith.  Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?” – I John 5:4 – 5.  So Christians do not have to fear the second death.

Revelation 20:6 states: “Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.” These are people who died for their faith in the Tribulation and are resurrected to enter into the Millennium.

Revelation 20:11 – 15 also mention the second death: “And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.  And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.  And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.  And death and hell were cast into the lake of fire. This is the second death.  And whosoever was not found written in the book of life was cast into the lake of fire.”

Revelation 21:7 & 8 states: “He that overcometh shall inherit all things; and I will be his God, and he shall be my son.  But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.”

The second death refers to the Lake of Fire where those who are separated from God by their sin will be for all eternity.  No “overcomer” will have a part in the second death.