What constitutes cross-chaining?
 

I recently went with a 12/25 cassette rather than the 13/27 I was riding. I did so not for any particular reason- it was what was in stock when I was in need. However, in doing so, my most used gear range has extended up to the larger sprockets, one sometimes two above mid-line while using the large chain ring (I have a standard double) How far beyond midline can one go without causing too much stress on the chain? May be relevent is that I have a medium length rear derailleur (not a short cage)

if it's a 9 or 10sp, then the chain doesn't suffer badly from cross chaining.

Is this a fact? I do have 10 sp. Not an issue, or major one at least? Good to know- less stress in my head over stress on the chain!

Yes, not as big of a deal. If you don't hear any rubbing, you should be fine. However, if you are getting faster, you could find yourself back in the middle range by using slightly larger gears :D

yup, 10sp and 9sp chains are very flexible and don't suffer from crosschaining like 5/6/7sp chains do.

What is cross chaining?

Cross-chaining means that you have your chain on the big chainring and the biggest cog or the smallest chainring and the smallest cog. On 5/6 speed bikes, the chains weren't very flexible, and running the chain at an extreme angle like that meant that the chain wouldn't mesh very well with the gears, leading to rapid wear of the chain.

Today, it's hard to get the derailleur's adjusted properly so that they run quietly in the big/big or small/small combos. And even if you do get the derailleur's adjusted, the chain will probably still be rubbing on something and cause a hellacious racket. So it's still not a great idea, but it's not a big no-no like it used to be.