seisin (n.)

also seizin, c. 1300, saisine, "possession as freehold, possession of land under rightful title," from Old French seisine, from saisir, seisir "sieze" (see seize (v.)). Originally it referred to the completion of the ceremony of feudal investiture (by which the tenant was admitted into his freehold). Extended sense of "ownership, possession" is from mid-14c.