Magic Circuit

The Magic Circuits (魔術回路 majutsu kairo) are the pseudo nervous system a magus has in his body. They are the infrastructure through which the lifeforce is turned into prana to render magecraft possible.

Magecraft is what uses prana to activate a "rule predetermined by the world". That rule requires a key (pass, spell, code), and if prana is the electrical current, the circuits are the pipeline that leads that current to the core system.

It is opened once, then the magus can turn it on and off at will. The switch responds to an image the user has of it: Emiya Shirou figures the firing hammer of a gun, and Tohsaka Rin has the image of a knife piercing the heart. This is a way of "opening" one, but there are various others to do so, such as sexual excitement or auto-mutilation.

Their number is predetermined at birth, and each magus in a lineage add up their own to the total. Shirou's number of Magic Circuits is surprisingly high, as he has 27. Rin has 40 main circuits, plus 30 sub circuits.

Source: Fate/Side Material encyclopedia entry (p.73), Tsukihime Dokuhon PlusPeriod dictionary entry (p.188)