[troubletb]

To me and this is just my opinion. A runt is the smallest by far. Not every litter has one. But they are most likely than not the one that was conceived last. Usually smaller and sometimes not always in health problems. If you have a litter of puppies that are (for example) 6 weeks old all weigh in around 1.5lbs and then there is one that weighs 12 oz, I would call that the runt.
Like I said this is just my OPINION!!