Revelation 20:6; NKJV
Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years.
Revelation 20:14; NKJV
Then Death and Hades were cast into the lake of fire. This is the second death.
Death is the separation of the spirit from the body.
"The Second Death" is separation from Jehovah God for all of eternity.
It is appointed unto all of us to die once (Hebrews 9:27). "The Second Death" has no power over those who are in Christ (Revelation 20:6, 14). Prepare for one; avoid the other.