Thayer's has a good definition of what death is. Much closer to the facts than the Merriam-Webster definition given earlier in this thread.
Title : Thayers Greek Definitions
Edition : Third
Copyright : Electronic Edition STEP Files Copyright © 1999, Parsons Technology, Inc.
G2288 θάνατος thanatos Thayer Definition:
1) the death of the body
1a) that separation (whether natural or violent) of the soul and the body by which the life on earth is ended
1b) with the implied idea of future misery in hell
1b1) the power of death
1c) since the nether world, the abode of the dead, was conceived as being very dark, it is equivalent to the region of thickest darkness, i.e. figuratively, a region enveloped in the darkness of ignorance and sin
2) metaphorically, the loss of that life which alone is worthy of the name
2a) the misery of the soul arising from sin, which begins on earth but lasts and increases after the death of the body in hell
3) the miserable state of the wicked dead in hell
4) in the widest sense, death comprising all the miseries arising from sin, as well physical death as the loss of a life consecrated to God and blessed in him on earth, to be followed by wretchedness in hell
Part of Speech: noun masculine
A Related Word by Thayers/Strongs Number: from G2348
Citing in TDNT: 3:7, 312
Total KJV Occurrences: 118
death, 115 Matt 4:16, Matt 10:21, Matt 15:4, Matt 16:28, Matt 20:18, Matt 26:38, Matt 26:66, Mark 7:10, Mark 9:1, Mark 10:33, Mark 13:12, Mark 14:34, Mark 14:64, Luke 1:79, Luke 9:26-27 (2), Luke 22:33, Luke 23:15, Luke 23:22, Luke 24:20, John 5:24, John 8:51-52 (2), John 11:4, John 11:13, John 12:33, John 18:32, John 21:19, Acts 2:24, Acts 13:28, Acts 22:4, Acts 25:11, Acts 25:25, Acts 26:31, Acts 28:18, Rom 1:32, Rom 5:10, Rom 5:12 (2), Rom 5:14, Rom 5:17, Rom 5:21, Rom 6:3-5 (3), Rom 6:9, Rom 6:16, Rom 6:21, Rom 6:23, Rom 7:5, Rom 7:10, Rom 7:13 (2), Rom 7:24, Rom 8:2, Rom 8:6, Rom 8:38, 1 Cor 3:22, 1 Cor 11:26, 1 Cor 15:21, 1 Cor 15:26, 1 Cor 15:54-56 (3), 2 Cor 1:9-10 (2), 2 Cor 2:16 (2), 2 Cor 3:7, 2 Cor 4:11-12 (2), 2 Cor 7:10, Phil 1:20, Phil 2:8 (2), Phil 2:27, Phil 2:30, Phil 3:10, Col 1:22, 2 Tim 1:10, Heb 2:9 (2), Heb 2:14-15 (3), Heb 5:7, Heb 7:23, Heb 9:15-16 (2), Heb 11:5, James 1:15, James 5:20, 1 John 3:14 (2), 1 John 5:16-17 (4), Rev 1:18, Rev 2:10-11 (2), Rev 2:23, Rev 6:8 (2), Rev 9:6 (2), Rev 12:11, Rev 13:3, Rev 18:8, Rev 20:6, Rev 20:13-14 (3), Rev 21:8
deadly, 2 Rev 13:3, Rev 13:12
deaths, 1 2 Cor 11:23