Tough Question.

Hmmm....
"Sustain: v. 1. to keep in existence; maintain or prolong 2. to provide sustenance for 3. to carry the weight of; to support 4. to endure; withstand 5. to comfort or encourage 6. to suffer (an injury, loss, etc.) 7. to uphold the validity of 8. to confirm; corrobate (Webster's New World Dictionary)"
I haven't been in the ecological activist pool for very long, so "sustainability" is actually still kind of a new term for me. In my perspective, as of right now, it just means the fight to keep the environment at a stable point, if not improving it --- and sustaining the balance between what we need to take from the earth and what we need to give back.
Getting back to the definition...not all of those numbers mean the same in correlation to this subject, but I tried to find connections anyways... :)
1. Well, we obviously need to keep what we still have of the many wonderful and precious things in our world in existence...there are far too many times when we take the air we breathe, the water we drink, and the materials we use for granted. 2., 5., 7.,& 8. just like we take from the world, we need to take care of it in return. 3. All this denotes a certain serious responsibility we have to the environment and sustainability is basically owning up to that responsibility. 4. & 6. If we hurt the earth, we hurt ourselves, even if we don't realize it. It's really for everyone's benefit that we make things sustainable, etc. That's just what I can assume from the word.