In Genesis 2:17, God’s warning “you will certainly die” (מֹות תָּמֽוּת) refers to the penalty or consequence of Adam and Eve’s sin, should they disobey God’s command. They had been given the ongoing privilege to “live forever” by accessing the Tree of Life (Gen 3:22 cf. 16), but this would be forfeited and their lives would be cut short by death—death as normally and universally understood; sometimes called “physical death.”
The main objection to this view is that Adam and Eve did not die “in the day” that they ate (Gen 2:17), if in fact ordinary death was in view. But this is to misunderstand the Hebrew idiom, as Walter Kaiser et al. explain:
It is just as naive to insist that the phrase “in the day” means that on that very day death would occur. A little knowledge of the Hebrew idiom will relieve the tension here as well. For example, in 1 Kings 2:37 King Solomon warned a seditious Shimei, “The day you leave [Jerusalem] and cross the Kidron Valley [which is immediately outside the city walls on the east side of the city], you can be sure you will die.” Neither the 1 Kings nor the Genesis text implies immediacy of action on that very same day; instead they point to the certainty of the predicted consequence that would be set in motion by the act initiated on that day. Alternate wordings include at the time when, at that time, now when and the day [when] (see Gen. 5:1; Ex. 6:28; 10:28; 32:34).
In other words, “you will certainly die” became true instantly, as a kind of death sentence or curse. But the timing of the death event is not specified in the warning. This is clear in the Aramaic translation of Genesis 2:17 found in Targum Jonathan, which suffices to show that at the time of Jesus this was viewed as ordinary death. It reads, “in the day that thou eatest thou wilt be guilty of death.”