TermMeaning
YKYear of the Kingdom, counted from Galifar’s founding
Five NationsAundair, Breland, Cyre (gone), Karrnath, Thrane: the old core of Galifar
GalifarThe lost united kingdom; also the gold coin
Dark LanternsBreland’s crown intelligence service
Sentinel MarshalsHouse Deneith’s cross-border lawmen
InspiredThe rulers of Riedra in Sarlona, said to be touched by spirits of dreams
KhyberThe underworld beneath Eberron; also “the Dragon Below”
SiberysThe ring of glittering light in the sky; also “the Dragon Above”
MourningThe catastrophe that destroyed Cyre
AberrantA dragonmark that belongs to no house

Sources: Eberron: Rising from the Last War; Exploring Eberron; Crowned Prophecy campaign notes (player-safe only).

From the Eberron Player Primer. Public knowledge and rumour only.