n. A substance trapped inside IC packages that enables them to function (also called 'blue smoke'; this is similar to the archaic 'phlogiston' hypothesis about combustion). Its existence is demonstrated by what happens when a chip burns up -- the magic smoke gets let out, so it doesn't work any more.
See smoke test, let the smoke out.
USENETter Jay Maynard tells the following story:
"Once, while hacking on a dedicated Z80 system, I was testing code by blowing EPROMs and plugging them in the system, then seeing what happened. One time, I plugged one in backwards. I only discovered that *after* I realized that Intel didn't put power-on lights under the quartz windows on the tops of their EPROMs -- the die was glowing white-hot. Amazingly, the EPROM worked fine after I erased it, filled it full of zeros, then erased it again. For all I know, it's still in service. Of course, this is because the magic smoke didn't get let out."
Compare the original phrasing of Murphy's Law.