1. adj. As yet unexplained, or too complicated to explain; compare automagically and (Arthur C.) Clarke's Third Law: “Any sufficiently advanced technology is indistinguishable from magic.” “TTY echoing is controlled by a large number of magic bits.” “This routine magically computes the parity of an 8-bit byte in three instructions.”
2. adj. Characteristic of something that works although no one really understands why (this is especially called black magic).
3. n. [Stanford] A feature not generally publicized that allows something otherwise impossible, or a feature formerly in that category but now unveiled.
4. n. The ultimate goal of all engineering & development, elegance in the extreme; from the first corollary to Clarke's Third Law: “Any technology distinguishable from magic is insufficiently advanced”.
Parodies playing on these senses of the term abound; some have made their way into serious documentation, as when a MAGIC directive was described in the Control Card Reference for GCOS c.1978. For more about hackish ‘magic’, see Appendix A. Compare black magic, wizardly, deep magic, heavy wizardry.