A type of
code that, in its
finished state, does exactly
what it was designed to do, under all possible scenarios of
underlying hardware and
software performing as they are
expected to do.
Note that what is bug-free code on one platform is not necessary bug-free code on any other platform, and the definition of bug-free code incorporates the platform it is designed to operate correctly on as a part of the design of what it was designed to do.
For example: If a piece of code is designed to only operate on Linux, then using Linux extensions to the C library is okay. However, if a piece of code is designed or intended to run on any platform other than Linux (say, Solaris, BSD, SCO, Alpha), the use of those extensions introduces bugs on those other platforms... and thus, the code is not bug-free code.