The Collaborative International Dictionary of English v.0.48:

Proprietorship \Pro*pri"e*tor*ship\, n.
   The state of being proprietor; ownership.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proprietorship
    n 1: an unincorporated business owned by a single person who is
         responsible for its liabilities and entitled to its profits
         [syn: proprietorship, proprietary]