The Keyblade is a type of legendary weapon originating from the Kingdom Hearts franchise and universe.
In the universe of Kingdom Hearts, there are two points in history that could be considered the origin of the Keyblade:
The first is a near-mythical historical weapon entitled the χ-blade (pronounced identically to 'Keyblade'). The χ-blade is the key that opens the door to the magical force that is Kingdom Hearts. It has no known clear point of conception in the series mythology, and neither does Kingdom Hearts; however, their first summoning is known in legend as the Keyblade War, an event in which thousands of people battled to create the χ-blade and claim Kingdom Hearts for themselves. The war caused the world to fall into darkness and eventually be destroyed, and the χ-blade broke into thirteen shards of darkness and seven of light, its pieces disseminating among the people. Though it is understood that the seven shards of light became the hearts of the Seven Princesses of Light, any battle between a force of pure light and a force of pure darkness can cause a new χ-blade to be forged.
The second are the blades forged during the Keyblade War, which are the first modern Keyblades. Forged in the image of the χ-blade, these keys were intended to be used to fight both the Heartless and the Keyblade War. and potentially to channel the true χ-blade. After the events of the war destroyed the original form of the world, many of these Keyblades were left to rust in the Keyblade Graveyard, and the surviving wielders turned Keyblade ownership into a secret rite of passage, taking apprentices from different worlds and training them to use its power wisely.
Innately, the Keyblade comes in three different varieties - Keyblades of the Realm of Light, Keyblades of the Realm of Darkness, and Keyblades comprised directly of people's hearts. While the second and third are very rarely seen, the first are the most common. These Keyblades are split into two categories themselves: Keyblades which those of the modern day have inherited, and Keyblades that those of the modern day have self-generated. The former are generally bestowed by current wielders in a ceremony where a worthy candidate holds the wielder's Keyblade. The latter requires the owner to have a designated weapon to which they are attached, around which the Keyblade forms; this weapon need not be physical.
The Keyblade is innately tied to the concept of a lock, both physical and metaphorical. The Keyblade is said to be able to open any door, and has demonstrably been capable of opening doors, chests, digital firewalls, and holes blocked by giant rocks, without regard for who originally placed them, with the only concern being the ability to face them as a person (so computer maintenance only takes place inside computers that can operate as worlds).
The Keyblade is also capable of opening and closing metaphysical locks - the most prominent being the Keyhole that protects the heart of each world, as well as the Gates that close off Gummi Routes between worlds. Certain Keyblades are used to extend this application to the locks on the hearts of humans, though it is still unsure whether this is only an inbuilt feature of these particular Keyblades, a high-level technique available to all Keyblades, or a combination of the two.
Keyblades are, obviously, also physical weapons, employed with similar use to swords or clubs. Despite the design of each Keyblade featuring blades of different thicknesses, all Keyblades seemingly alternate between being sharp or blunt objects depending on the object encountered - Keyblades are apparently blunt when attacking physical enemies but sharp when encountering objects, as a general rule.
In regards to magical and non-physical techniques, Keyblades can serve as conduits for combat magic of all elements. Some characters are able to employ Shotlocks, a type of lock-on magical blast. In Birth by Sleep, the user's signature Keyblade can be transformed into a Glider form that allows for solo travel through Gummi Space; in Kingdom Hearts II Final Mix and Kingdom Hearts III the Keyblade can take many other wildly different forms for the purpose of combat, including guns and chariots.
The Keyblade comes in many different designs, usually representative of a world or a specific person. While each person's Keyblade tends to have a unique design, its design can be changed by use of a Keychain. A single Keyblade may have several designs that are default to it depending on its wielders, and a single design may also pass between multiple Keyblades, but all Keyblades have specific keychains with a notable pair of exceptions - the Keyblade of people's hearts, which is implied to be incomplete; and the Keyblades in the Keyblade Graveyard, which left behind by their deceased owners and are referred to as 'lifeless'.
Due to its original purpose as a standard military weapon, Keyblades are not unique; however, despite originally being widespread, the death of multiple owners without a rite of passage had controlled the number of Keyblades down to a significantly smaller number. Nonetheless, this number comprises many of the protagonists of the Kingdom Hearts series.
[to be inserted - basically the entire Kingdom Hearts continuity in release order by year]