The Linker Core (リンカーコア Rinkā Koa) is an organ in a mage's (knight's) body which works to link and manifest Mana generated within themselves. The power potential of a mage thus depends on the capacity of their Linker Core, and currently any analysis of a mage's abilities are done by examining the Linker Core.
In computer science, a linker is a program that transforms compiled program code into executable programs; similarly, the Linker Core transforms a mage's energy calculations into a functional spell. Also, "core" is another term for a central processing unit.
A mage's Linker Core can be forcibly removed from their bodies but it apparently regenerates with time. Nanoha Takamachi had her Linker Core stolen by Shamal in A's but still managed to fire a Starlight Breaker immediately afterwards, most likely because the Starlight Breaker had already been prepared and merely needed to be fired. Nanoha was unable to cast any magic at all for the next few days. In fact, the Book of Darkness relied on the Wolkenritter stealing others' Linker Cores, as they apparently also contain magical knowledge of the respective mage, i.e. spells that the Book needs to fill its 666 pages.
Species that are known to possess Linker Cores are:
- Humans (only mages)
- Wolkenritter (and probably other living programs like Materials, too)
- Unison Devices
- Artificial Mages
- Combat Cyborgs
Familiars are said to "share one heart" with their masters, which may mean that they effectively use their master's Linker Core for their magic. This is indirectly supported by the fact that familiars and their masters share all their knowledge of magical spells.