Death (means a “separation”)

Death of the body:

It is separation of the body and Spirit

James 2:26 

As the body without the spirit is dead, so faith without deeds is dead.

Ecclesiastes 12:7 

And the dust returns to the ground it came from, and the spirit returns to God who gave it.
Death entered the world because of sin:

Genesis 2:17 

But you must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die.”

Romans 5:12 

Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned—

Everyone dies through Adam:

1 Corinthians 15:21-22 

21 For since death came through a man, the resurrection of the dead comes also through a man. 

22 For as in Adam all die, so in Christ all will be made alive.

Appointed unto man to die:

Hebrews 9:27 

Just as man is destined to die once, and after that to face judgment,

Ecclesiastes 8:8 

No man has power over the wind to contain it; so no one has power over the day of his death. As no one is discharged in time of war, so wickedness will not release those who practice it.

Spiritual Death

It is separation from God because of sins:

Isaiah 59:1-2

1 Surely the arm of the LORD is not too short to save, nor his ear too dull to hear.

2 But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear.