"For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord," (Rom. 6:23).
This verse can be interpreted to mean both spiritual and physical death. However, notice that it speaks of death and life. It says that "the wages of sin is death" and then speaks of "eternal life." Eternal life means that a person has a saving relationship with God and will not face eternal damnation. Consider what Jesus said in Matt. 25:46, "And these will go away into eternal punishment, but the righteous into eternal life.” Also John 3:16, "For God so loved the world, that He gave His only begotten Son, that whoever believes in Him should not perish, but have eternal life." In these two verses, along with many others, eternal life is contrasted with eternal punishment and perishing. Therefore, it would seem that the primary meaning of the text would be that the phrase, "the wages of sin is death," is referring to spiritual death which would be eternal separation from God, damnation.
However, we also know that sin has brought physical death into the world. When Adam and Eve partook of the forbidden fruit, they eventually died physically. Sin, which is breaking the law of God (1 John 3:4), brings both physical (Rom. 5:12) and spiritual death (Isaiah 59:2). So, Romans 6:23 can legitimately be interpreted to include both spiritual and physical death when it speaks of "the wages of sin."