[poss. from C. S. Lewis's "Narnia" books]
n. An awesomely arcane technique central to a program or system, esp. one not generally published and available to hackers at large (compare black art); one that could only have been composed by a true wizard. Compiler optimization techniques and many aspects of OS design used to be deep magic; many techniques in cryptography, signal processing, graphics, and AI still are. Compare heavy wizardry. Esp. found in comments of the form "Deep magic begins here...".
Compare voodoo programming.