In Targum Neofiti and the fragments, the "second death" is the death that the wicked die.
Targum Isaiah has three occurrences. The first is 22:14 where the Aramaicparaphrases the Hebrew as "This sin will not be forgiven you until you die the second death." The final two examples are from Targum Isaiah 65 which sets the scene for an apocalyptic final battle. Targum Isaiah 65:6 paraphrases the Hebrew in line with the interpretation of the penultimate verse of the Hebrew Isaiah, found in the Gospel of Mark, where "their worm does not die" is equated with Gehinnom. Here both Targum Isaiah and Gospel of Mark supply the term "Gehinnom", where Hebrew Isaiah simply concludes with the heaps of corpses following the last battle where "their worm does not die", making no further eschatological extension into resurrection and judgment.
Targum Jeremiah 51:17 has the Aramaic "they shall die the second death and not live in the world to come", which appears to depart from the other Targum uses in not being explicit that the second death is after resurrection but may instead be an exclusion from resurrection.
The majority reading of Targum Psalm 49:11 has the Aramaic translation "For the wise see that the evildoers are judged in Gehinnom". However, several manuscripts, including Paris No.10, Montefiore No.7, and Targum of Salomos 113 have the variant Aramaic translation "He sees men wise in wickedness, who die a second death, and are judged in Gehinnom".
The term "second death" occurs four times in the New Testament, specifically in Revelation 2:11, 20:6, 20:14 and 21:8. According to Revelation 2:11 and 20:6, those who overcome the devil's tribulation, are holy and have part in the first resurrection will not experience the second death. Revelation 20:14 and 21:8 then identify the second death with the lake of fire. In we read: "s for the cowardly, the faithless, the polluted, the murderers, the fornicators, the sorcerers, the idolaters, and all liars, their place will be in the lake that burns with fire and sulfur, which is the second death."
When people are saved, they are not subject to the second death. They die only the first, earthly death. However, an unsaved person will experience two deaths: the first death and then after the resurrection the second death which is usually interpreted as everlasting torment or everlasting destruction. The traditional view of everlasting torment was elucidated by Lactantius: Christian universalists reject the notion of endless torment and therefore offer different interpretations. For instance, Gregory of Nyssa understood the second death as a cleansing, albeit painful process. He wrote that "those still living in the flesh must as much as ever they can separate and free themselves in a way from its attachments by virtuous conduct, in order that after death they may not need a second death to cleanse them from the remnants that are owing to this cement of the flesh". Annihilationists and conditionalists, including all Seventh-day Adventists and Jehovah's Witnesses, and others in many denominations, also oppose the idea of eternal suffering but believe that the second death is an actual second death and that the bodies and souls condemned to it after the final judgment will be utterly destroyed.
The Mandaeans believe that the souls which could not be purified inside of demon Ur would get destroyed along with him at the end of days, so they die the second death.