Dictionary.net

[possibly from C. S. Lewis's "Narnia" books] An awesomelyarcane technique central to a program or system, especiallyone neither generally published nor available to hackers atlarge (compare black art); one that could only have beencomposed by a true wizard. Compiler optimisation techniquesand many aspects of OS design used to be deep magic; manytechniques in cryptography, signal processing, graphics, andAI still are. Compare heavy wizardry. Especially found incomments of the form "Deep magic begins here.". Comparevoodoo programming.