gartel! The gartel is the "waistline" in the middle of the etrog giving it that hourglass figure. The term gartel comes from the Yiddish and means 'belt'. Hassidim wear a black gartel over their long jackets to signify the separation of their upper body where the heart and mind are and the lower body where the more animalistic functions reside.
So when Hasidim look for an especially beautiful (mehudar) etrog they look for one with a gartel. They also had a tradition that the etrog with the gartel is reliably held to fulfill the requirement of growing on a tree that was not grafted. (Citrus trees like oranges are usually grafted to lemon trees because they are stronger.)
It seems that there is even some scientific basis for the theory that the viroids which cause the gartel help prove that this species of etrog is genetically pure. See the Wikipedia here.
Happy Succot Everyone !!!!!
*A big thank you to David for helping with the background for this post!