Abad (אָבַד) means to perish, vanish, be destroyed, or be lost. It denotes total ruin or extinction — things that cease to exist, people who die violently, or nations that are wiped out. The word appears approximately 185 times in the OT across multiple conjugations, including the Hiphil (to cause to perish, to destroy) and the Piel (to destroy utterly).
Its antonym is chayah (H2416, to live) and it stands in stark contrast to concepts of life, salvation, and covenant preservation.
Abad describes both physical death and theological ruin. In Deuteronomy, it is a covenantal term — Israel would abad if they turned from YHWH (Deut. 8:19). This is not mere national extinction but a theological judgment: those outside God's covenant perish. The Psalms use it of the wicked whose way "will perish" (Psalm 1:6).
In the New Testament, the cognate concept is captured by the Greek apollumi (G622), which occurs famously in John 3:16: "that whoever believes in him should not perish." The gospel is precisely the remedy for abad — God acts to save what would otherwise be utterly lost.