A program (read: simple spell) used in cyberspace.
A wizard writes and compiles the program code in advance, and activates it by passing magic through it.
There are two types of Codecast. External Codecasts are one time use consumable Codecasts, while Internal Codecasts are installed into the body… or rather the spiritron structures [that make up one’s body in cyberspace].
Codecasts that have been installed are very powerful, however seeing as they change the fundamental nature of the user, not many wizards choose to use them. Codecasts implemented through the use of a Mystic Code fall somewhere in the middle [in terms of power and effects on the user].

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA