A long long time ago, in an office building a couple of miles away, I was asked this question in an interview. I’m not sure how much they cared about the answer, but mine was more glib than it was useful (I got the job anyway). I misquoted Einstein and said that good code was code that was as simple as possible but no simpler.
This was not a very satisfactory answer to me, though I got the job.
For, uh, reasons this question has been on my mind recently and I think I’ve come up with an answer that satisfies me:
Good code is code that I am unlikely to need to modify but easily could if I wanted to.
Of course, good code is probably only roughly correlated with good software.