precarium

1 Roman, civil, & Scots law a :  something granted or lent to be returned or redelivered at the will of the grantor b :  a contract making a loan or grant upon such terms or the tenure by which it is held :  a tenancy at will

2 medieval European feudal law :  any of various estates or tenures that grew out of the original precarium characterized by more or less uncertainty or limitation of the duration and arduousness of the conditions of tenure — compare benefice