Seizin

Seisin (or seizin) denotes the legal possession of a feudal fiefdom or fee, that is to say an estate in land. It was used in the form of "the son and heir of X has obtained seisin of his inheritance", and thus is effectively a term concerned with conveyancing in the feudal era. The person holding such estate is said to be "seized of it", a phrase which commonly appears in inquisitions post mortem (i.e. "The jurors find that X died seized of the manor of ..."). The monarch alone "owned" all the land of England by his allodial right and all his subjects were merely his tenants under various contracts of feudal tenure.

Seisin comes from Middle English saysen, seysen, in the legal sense of to put in possession of, or to take possession of, hence, to grasp, to seize. The Old French variations seisir, saisir, are from Low Latin sacire, generally referred to the same source as Gothic satjan, Old English settan, to put in place, set.

Seisin is believed to have been applicable only to freehold tenures, that is to say a tenure for a term of life, which was heritable, on condition of payment of the appropriate feudal relief to the overlord. A "freeman" was a man who held by freehold tenure, and thus freehold tenure was anciently said to be the only form of feudal land tenure worthy to be held by a free man.Tenure, and the variety thereof, was the very essence of feudal society and the stratification thereof, and the possession of a tenure (i.e., holding, from Latin teneo "to hold") was legally established by the act of seisin.

Seisin used in the normal course of events is of two kinds, "in law" and "in deed". Each carries with it a differing strength of tenure. It came to be said that later that in the conveyance of a fee by deed of feofment there must be livery of seisin.

...
Wikipedia