I guess the best definition is something you don't normally see. I would say anything with an edition size of 100 or less. For example, most if not all of my wants are what I consider rare or maybe super rare. I just acquired an LE 2 pin. I also have an LE 12 framed set and an LE 25 framed set.
However, it can't just be based on edition size. For example the haunted mansion dangle is LE 1000, but I would say that is rare. It was released in or before 1999 and most haven't seen the light of day since. Some on my want list were released in 1964 and earlier.
Hope this helps.
