Who are the Seven Abdalites, and what is a brief overview of their history?

The term "al-Abadilah" is applied to four of the younger Companions who amassed vast knowledge: Abdullah ibn Abbas, Abdullah ibn al-Zubayr, Abdullah ibn Umar, and Abdullah ibn Amr ibn al-As. Abdullah ibn Mas'ud is not among them due to his advanced age and early demise, unless the intention is to bring their number to seven. Abdullah ibn Abdullah ibn Ubayy ibn Salul and Abdullah ibn Waqid can also be added to them.