Formal or legal term for the person who owns something. Often interchangeable with 所有者, but 所有主 can emphasize the person as the master or holder.
See also: 所有者
Who is the owner of this land?
You cannot enter without the owner's permission.
Compound of 所有 (shoyū, 'ownership') and 主 (nushi, 'master' or 'owner').