The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

lobbyist
    n 1: someone who is employed to persuade legislators to vote for
         legislation that favors the lobbyist's employer

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "lobbyist":
   Rasputin, Svengali, VIP, access, bad influence, big wheel, court,
   eminence grise, five-percenter, friend at court, good influence,
   gray eminence, heavyweight, hidden hand, influence,
   influence peddler, influencer, ingroup, key, kingmaker,
   legislative lobby, lobby, lobbyer, lords of creation,
   man of influence, manipulator, open sesame, parliamentary agent,
   powers that be, pressure group, registered lobbyist,
   sinister influence, special interests, special-interest group,
   special-interest lobby, the Establishment, very important person,
   wheeler-dealer, wire-puller