Definition - What does Seisin mean?
Seisin is a term from Old English that refers to both ownership and possession of real property.
In historical England, the sovereign was considered to own much of the land, and people were just "in possession of it." So, someone who had seisin of land had both possession of it as well as ownership of it.
Justipedia explains Seisin
Although the word "seisin" has a historical context, it still has relevance today. In the modern era, it simply means that a person has full ownership, including the title to a piece of real property.
If a person has full ownership, then they possess the property and also have the ability to control it. This means that they can sell it, grow crops on it, mine it for minerals, etc.