Etymology

Noun

1. The perpetual, inalienable possession of lands by a corporation or non-personal entity such as a church.
   • 1824, Charter of Incorporation of the Royal Asiatic Society of Great Britain and Ireland,

     [W]e do hereby grant our especial license and authority unto all and every person to grant sell alien and convey in mortmain unto and to the use of the said Society and their successors

   • 1900, Frederic William Maitland, "The Corporation Sole", Law Quarterly Review, v. 16,

     Though in truth it was the law of mortmain which originally sent the founders of chantries to seek the king's licence

2. A strong and inalienable possession.
   • 1770, Edmund Burke, Thoughts on the Present Discontents, and Speeches,

     ; and some part of that influence [of the government], which would otherwise have been possessed as in a sort of mortmain and unalienable domain, returned again to the great ocean from whence it arose,