The Collaborative International Dictionary
Seisin \Sei"sin\, n.
See Seizin.
--Spenser.
Wiktionary
n. 1 (context legal common law historical English) A feudal term for an entitlement to a freehold estate with a right to immediate possession; still used in technical discussions of real property law today. 2 (context obsolete English) The act of taking possession. 3 (context obsolete English) The thing possessed; property.
Wikipedia
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.
Usage examples of "seisin".
The copyhold was also subject to a variety of grievous taxes, which the lord had the privilege, upon many occasions, of imposing - such as aids, reliefs, primer seisin, wardship, escheats for felony and want of heirs, and many more, altogether so exorbitant and oppressive as often totally to ruin the tenant and rob him of almost all interest in his property.
The copyhold was also subject to a variety of grievous taxes, which the lord had the privilege, upon many occasions, of imposing - such as aids, reliefs, primer seisin, wardship, escheats for felony and want of heirs, and many more, altogether so exorbitant and oppressive as often totally to ruin the tenant and rob him of almost all interest in his property.