The Hebrew noun ad means perpetuity, eternity, or unending continuity. Often paired with prepositions — le-ad (forever), ad-olam (forever and ever) — it expresses the endless duration of God's reign, covenant, and salvation. Unlike olam, ad points strictly forward into endless future.
Isaiah 9:6 gives the Messiah the title 'Everlasting (Ad) Father' — Avi Ad — Father of Eternity. Psalm 9:7: 'The LORD reigns forever (le-ad).' Ad is the horizon that never arrives — God's kingdom simply does not end. It proclaims the inexhaustible, forward-moving eternality of the divine reign.