Definitions for tanistry
A former form of tenure, in Scotland and Ireland, in which land was held for life and then passed to an elected member of the same family
in Ireland, a tenure of family lands by which the proprietor had only a life estate, to which he was admitted by election
The Nuttall Encyclopedia
a method of tenure which prevailed among the Gaelic Celts; according to this custom succession, whether in office or land, was determined by the family as a whole, who on the death of one holder elected another from its number; the practice was designed probably to prevent family estates falling into the hands of an incompetent or worthless heir.