Deep Magic

Definitions

Code or technical knowledge so arcane and poorly documented that only one or two people understand it. Often preceded by a comment like '// Here be dragons.' A reference to both the Jargon File and C.S. Lewis's Narnia.
In plain English: Code or knowledge so obscure and poorly documented that only one or two people in the world actually understand how it works.
In organizational context, deep magic is institutional knowledge that exists only in one person's head — the undocumented configuration choices, workarounds, and historical context that explain why systems behave the way they do. A close cousin of tribal knowledge.
Example: 'The deployment script has a 30-second sleep in the middle. Nobody knows why. That's deep magic from 2019 — the person who added it left two years ago.'
Source: organizational / tribal knowledge