Keyblade: A key-shaped weapon that can vanquish the darkness and free the hearts from the Heartless. It can also unlock any lock, including the Keyholes to the hearts of worlds. They were originally weapons made to fight over Kingdom Hearts, but Keyblade-wielders today use them to guard the realm of light instead. You can change the shape and abilities of a Keyblade by equipping a keychain to it, and they can also be transformed into vehicles for traversing the Lanes Between. Some Keyblades belong to the realm of light, and some to the realm of darkness, but all that difference amounts to is which side of Kingdom Hearts' door it needs to be on to close it.
Lanes Between: The paths that allow travel between worlds. They're opened by Keyblade-bearers using the power of their Keyblades.
REALM OF LIGHT: People live in this light-filled realm. Everything was connected long ago, but after stability decayed, the realm was divided into smaller worlds. Every world is covered by an invisible wall, making it impossible to travel to other worlds without special means. Besides which, prancing between worlds is expressly forbidden, as it may mess with the natural order of each one. Every world has its own heart, and if you open the door that leads to it, darkness will cause the walls around the world to shatter. Those fragments rain down as a meteor shower, and they can be used as ingredients for Gummi Ships.
REALM OF DARKNESS: Located on the opposite side of the realm of light, no humans live there, and it's flooded with Heartless. The realm of darkness and realm of light are connected with the Door to Darkness and the Door to Light. The Door to Darkness can only be used by beings of darkness, and the Door to Light can only be used by residents of light.
REALM BETWEEN: It exists between the light and the dark. The number of worlds here is scarce in comparison to the realm of light. And because of how unstable it is, corridors of darkness are prone to open sporadically.
Unions: Unions are factions in the world of Kingdom Hearts Unchained x. There are five Unions in total, each led by one of the five chosen apprentices of the Master of Masters, the
Foretellers—Foreteller Invi's
Anguis, Foreteller Gula's
Leopardos, Foreteller Ira's
Unicornis, Foreteller Aced's
Ursus, and Foreteller Ava's
Vulpes. The goal of each Union is to defend the worlds from the
Heartless, collect light in form of
Lux and compete with the other Unions in those categories in different daily and weekly challenges.
Fortellers: The Foretellers are a group of Keyblade Masters that appears
Kingdom Hearts χ and consists of
Invi,
Gula,
Ira,
Aced, and
Ava. Together, the Foretellers lead the five
Unions and protect the worlds from
darkness.
Heartless: When a person loses their heart to darkness, the darkness in that heart is made real, and a being of darkness is born in the darkness. They lack intelligence and reason, but they yearn for darkness and go after the hearts of the worlds. They reproduce by stealing more hearts. Evil groups tend to use the Heartless for their own schemes.
PUREBLOOD HEARTLESS = Heartless that occur naturally. It's possible to free hearts when they're defeated by the Keyblade. The realm of darkness is home to an abundance of Purebloods.
EMBLEM HEARTLESS = Artificially manufactured Heartless that Ansem the Wise's apprentices made during their experiments. They bear a crest on their bodies that contains Xehanort's "X" recusant sigil.
Nobodies: The opposite of the Heartless known as "non-beings." When the owner of a strong heart becomes a Heartless, a Nobody is born. Most of them have the appearance of white ghosts, but if the owner of an especially strong heart becomes a Nobody, they can maintain their human form. While a Heartless is a heart lost to darkness, a Nobody is the body and soul that's separated from their heart. They're emotionless, instead using memory and intellect to act as a group.
Dream Eaters: Monsters that haunt the realm of sleep and eat dreams. They can appear as Nightmares, which induce bad dreams, or Spirits, which can consume nightmares.
The Ranking Parade: Every Monday morning in Daybreak Town, the keyblade wielders and Foretellers hold a special parade to celebrate the achievements of the wielders and Unions who did the best job of collecting lux during the past week. The top five best wielders are paraded around town and hailed as heroes, even given great rewards for their efforts.
Most of the above terms will apply to Kingdom Hearts: Daybreak, but if you're interested in reading more of them to learn more about the series lore
see here.