1. A person who knows how a complex piece of software or hardware works (that is, who groks it); especially someone who can find and fix bugs quickly in an emergency. Someone is a hacker if he or she has general hacking ability, but is a wizard with respect to something only if he or she has specific detailed knowledge of that thing. A good hacker could become a wizard for something given the time to study it.
2. A person who is permitted to do things forbidden to ordinary people; one who has wheel privileges on a system.
3. A Unix expert, especially a Unix systems programmer. This usage is well enough established that "Unix Wizard" is a recognised job title at some corporations and to most headhunters.
See guru, lord high fixer. See also deep magic, heavy wizardry, incantation, magic, mutter, rain dance, voodoo programming, wave a dead chicken.