What is Impropriation?

Legal Definition
Impropriation, a term from English ecclesiastical law, was the destination of the income from tithes of an ecclesiastical benefice to a layman. With the establishment of the parish system in England, it was necessary for the properties to have an owner. This was the parochianus or parson/rector who was sustained by the benefice income while providing personally for the cure-of-souls. The parson was technically a corporation sole. With the passage of time, the benefice came to be considered a piece of property whose holder could discharge the spiritual responsibilities by a deputy and many were appropriated by monasteries or other spiritual corporations. These were bound to provide for a cleric for the cure of souls in the parish but could use any excess income as they pleased. The deputy was often known as the 'vicar'.

Impropriation was similar except that the recipient was a layman or secular corporation who was obliged to provide a cleric to serve the parish and for his maintenance. After 1200, no layman could have a cure of souls but grants were still occasionally made. When the monastic properties passed into lay hands at the Reformation, many appropriations were converted into impropriations, and in 1603 of a total 9284 benefices an estimated 3489 were in the hands of impropriators or lay rectors. By custom, they were obliged to maintain the chancel in good repair.
-- Wikipedia
Legal Definition
Eccl. law. The act, of employing the revenues of a church living to one's own use; it is also a parsonage or ecclesiastical living in the hands of a layman, or which descends by inheritance. Techn. Dict. h. t.
-- Bouviers Law Dictionary
Legal Definition
In ecclesiastical law. The annexing an ecclesiastical benefice to the use of a lay person, whether individual or corporate, in the same way as appropriation is the annexing of any such benefice to the proper and perpetual use of some spiritual corporation, whether sole or aggregate, to enjoy forever. Brown.
-- Black's Law Dictionary
Legal Definition
The investiture of a benefice in a layman.
-- Ballentine's Law Dictionary