I've got a little different outlook than Haz, I agree with all his SHTF but throw Katrina and other local/regional disasters into that category as well. I guess the outlook difference for Katrina and such is both the size of the impact and the length of time impacted. I know the impact was more or less permanent for a number of people, but 30 miles away from New Oreleans things were pretty much back to normal when electricity returned (except for the social plaque that spread from New Orleans and increased crime in the areas of relocation).
For TEOTWAWKI my thought is that it would be some kind of transitional breakdown or change that leads to permanent changes (social and physical) that is a nationwide or global in impact. Like Haz's comment but also widespread civil unrest caused by, say, a currency failure that leads to the end of the current status quo and lasts until a happy equilibrium is reached. Not that the happy equilibrium is much better, but that some benevolent (more or less) structure or order is in place. TEOTWAWKI is a transitional time from one from one period of normalcy to another period of normalcy that may last for a long time but leads to some type of equilibrium....just my 2 cents.