A substance trapped inside
integrated circuit 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
Electing a Pope,
smoke test.
Usenetter Jay Maynard tells the following story:
"Once, while hacking on a dedicated
Zilog 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
backward. I only discovered that *after* I realised 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.