Big, ambitious goals you set before you know how you’ll achieve them (or even if they are possible) to force you to think in terms of revolution and not just evolution.
Stretch Goals are those you set out at the beginning of an endeavor that seem like they’re almost impossible to achieve, but would result in a “resounding success” if they were met. Setting stretch goals is a good way to avoid keeping your ambitions too low in an effort to ensure you don’t fall short of your SMART Goals.
These are tied closely in with what John Doerr called “Big, Hairy, Audacious Goals” in Measure What Matters time and time again. The point of OKRs was that you weren’t really supposed to achieve all of them, because that meant you set your ambitions too low.