The place where you put things when your pdl is full. If you don't have one and too many things get pushed, you forget something. The overflow pdl for a person's memory might be a memo pad. This usage inspired the following doggerel:

 Hey, diddle, diddle
 The overflow pdl
   To get a little more stack;
 If that's not enough
 Then you lose it all,
   And have to pop all the way back.
                               --The Great Quux

The term pdl seems to be primarily an mitism; outside MIT this term is replaced by "overflow stack".