1. [n]  an unincorporated business owned by a single person who is responsible for its liabilities and entitled to its profits
2. [adj]  protected by trademark or patent or copyright; made or produced or distributed by one having exclusive rights; "`Tylenol' is a proprietary drug of which `acetaminophen' is the generic form"

PROPRIETARY is a 11 letter word that starts with P.

1. \Pro*pri"e*ta*ry\, n.; pl. {Proprietaries}. [L.
   proprietarius: cf. F. propri['e]taire. See {Propriety}, and
   cf. {Proprietor}.]
   1. A proprietor or owner; one who has exclusive title to a
      thing; one who possesses, or holds the title to, a thing
      in his own right. --Fuller.
   
   2. A body proprietors, taken collectively.
   
   3. (Eccl.) A monk who had reserved goods and effects to
      himself, notwithstanding his renunciation of all at the
      time of profession.
   
   

2. \Pro*pri"e*ta*ry\, a. [L. proprietarius.]
   Belonging, or pertaining, to a proprietor; considered as
   property; owned; as, proprietary medicine.
   
   {Proprietary articles}, manufactured articles which some
      person or persons have exclusive right to make and sell.
      --U. S. Statutes.
   
   

1. In marketroid-speak, superior; implies a product imbued with exclusive magic by the unmatched brilliance of the company's own hardware or software designers.

2. In the language of hackers and users, inferior; implies a product not conforming to open-systems standards, and thus one that puts the customer at the mercy of a vendor who can inflate service and upgrade charges after the initial sale has locked the customer in.

[jargon file]