Hebrew berith chadashah (Jeremiah 31:31), Greek diathēkē kainē. Jeremiah prophesies a day when YHWH will make a new covenant with Israel and Judah, "not like the covenant I made with their fathers" — one marked by four features: (1) the law written on hearts instead of stone (internalized); (2) all parties personally knowing the LORD (democratized); (3) sins remembered no more (forgiven); (4) unbreakable because God guarantees both sides (unilateral). At the Last Supper Jesus lifts the cup and says, "This cup is the new covenant in my blood" — He is the mediator inaugurating what Jeremiah saw at a distance.
The New Covenant reframes everything. The Old Covenant at Sinai was bilateral: "Do this and live; break this and die." Israel broke it repeatedly, and the prophets document the fallout — exile, famine, abandonment. The New Covenant is unilateral in its guarantees (God fulfills both sides in Christ) while still calling for faith. Its benefits are: justification (sins remembered no more, Hebrews 8:12), regeneration (law on the heart, not on tablets), indwelling Spirit (Ezekiel 36:26-27 — the companion prophecy), universal priesthood of believers (all know Him, from the least to the greatest), and eternal security (an everlasting covenant, Hebrews 13:20). Hebrews 8-10 is the sustained NT exposition. Dispensationalists see some New Covenant blessings held in reserve for a future national Israel; covenant theology sees the Church fully included now. Both agree: the New Covenant is ratified in Christ's blood and is currently operative for all who trust Him.