1. [n]  an expert in parliamentary rules and procedures
2. [n]  an elected member of the British Parliament: a member of the House of Commons

PARLIAMENTARIAN is a 15 letter word that starts with P.

1. \Par`lia*men*ta"ri*an\, a.
   Of or pertaining to Parliament. --Wood.
   
   

2. \Par`lia*men*ta"ri*an\, n.
   1. (Eng. Hist.) One who adhered to the Parliament, in
      opposition to King Charles I. --Walpole.
   
   2. One versed in the rules and usages of Parliament or
      similar deliberative assemblies; as, an accomplished
      parliamentarian.