Source : Webster's Revised Unabridged Dictionary (1913)

Lobbyist \Lob"by*ist\, n.
   A member of the lobby; a person who solicits members of a
   legislature for the purpose of influencing legislation.
   [U.S.]

Source : WordNet®

lobbyist
     n : someone who tries to persuade legislators to vote for bills
         that the lobbyists favor