Nickname of people who can turn mysteries that are not magecraft, things that are impossible, possible.
In the past when civilization was less developed, most magi were magicians.
As civilization developed, the impossible became possible, and many magic devalued into magecraft. Mysteries, in the face of reality, tasted great defeat.
———However, even among those, there still exists miracles unreachable by humans.
Magi who can realize those mysteries are called “magicians,” bearing with them fear and respect, admiration and jealousy.
In the world of Kara no Kyoukai, there are only five magicians.

Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary