Sustainable?

I'd like some genius to define sustainable. Could we count something that we can keep doing for 100 billion years - beyond the death of the Universe as we currently understand such things - as sustainable? How about a billion years? Or will a million suffice? A thousand? A hundred? Until the next innovation comes along?

Anyway. If any of my readers can answer this question in the comments I'd be much obliged.

M. Simon's e-mail can be found on the sidebar at Space-Time Productions [1].