Death is understood two main ways in the Bible. First, it is used to describe the cessation of life. This can be physical or spiritual. Second, death is used in reference to those who are in a life of sin. This refers to their eternal separation from God as a result of sin (Isaiah 59:2)--in a conscious state of damnation without hope (1 Thess. 4:13, Rev. 20:10, 14, 15).
Death to humans is unnatural. When God created Adam and Eve, death was not part of the created order. It was not until they sinned that death entered the scene (Rom. 5:12, 6:23). Death will be destroyed when Christ returns and the believers receive their resurrected bodies.