The Collaborative International Dictionary of English v.0.48:

Lobby \Lob"by\, n.; pl. Lobbies. [LL. lobium, lobia, laubia, a
   covered portico fit for walking, fr. OHG. louba, G. laube,
   arbor. See Lodge.]
   1. (Arch.) A passage or hall of communication, especially
      when large enough to serve also as a waiting room. It
      differs from an antechamber in that a lobby communicates
      between several rooms, an antechamber to one only; but
      this distinction is not carefully preserved.
      [1913 Webster]

   2. That part of a hall of legislation not appropriated to the
      official use of the assembly; hence, the persons,
      collectively, who frequent such a place to transact
      business with the legislators; hence: any persons, not
      members of a legislative body, who strive to influence its
      proceedings by personal agency; a group of lobbyists for a
      particular cause; as, the drug industry lobby. [U. S.]
      [1913 Webster +PJC]

   3. (Naut.) An apartment or passageway in the fore part of an
      old-fashioned cabin under the quarter-deck.
      [1913 Webster]

   4. (Agric.) A confined place for cattle, formed by hedges.
      trees, or other fencing, near the farmyard.
      [1913 Webster]

   Lobby member, a lobbyist. [Humorous cant, U. S.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lobby \Lob"by\, v. i. [imp. & p. p. Lobbied; p. pr. & vb. n.
   Lobbying.]
   To address or solicit members of a legislative body in the
   lobby or elsewhere, with the purpose to influence their
   votes; in an extended sense, to try to influence
   decision-makers in any circumstance. [U.S.] --Bartlett.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lobby \Lob"by\, v. t.
   To urge the adoption or passage of by soliciting members of a
   legislative body; as, to lobby a bill; -- also used with the
   legislators as object; as, to lobby the state legislatuire
   for protection. [U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

lobby
    n 1: a large entrance or reception room or area [syn:
         anteroom, antechamber, entrance hall, hall,
         foyer, lobby, vestibule]
    2: the people who support some common cause or business or
       principle or sectional interest
    3: a group of people who try actively to influence legislation
       [syn: lobby, pressure group, third house]
    v 1: detain in conversation by or as if by holding on to the
         outer garments of; as for political or economic favors
         [syn: lobby, buttonhole]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "lobby":
   Rasputin, Svengali, VIP, access, act on, advocate, approach,
   bad influence, bear upon, big wheel, blandish, cajole, call on,
   call upon, coax, constitute, corridor, court, decree, draw,
   draw on, eminence grise, enact, enact laws, entrance,
   entrance hall, entranceway, entry, entryway, exert influence,
   exhort, filibuster, five-percenter, foyer, friend at court,
   galilee, get cozy with, get the floor, good influence,
   gray eminence, hall, hallway, have the floor, heavyweight,
   hidden hand, high-pressure, importune, influence,
   influence peddler, influencer, ingroup, insist, insist upon,
   jawbone, key, kill, kingmaker, lead on, legislate,
   legislative lobby, lobby through, lobbyer, lobbyist, logroll,
   lords of creation, magnetize, make advances, make overtures,
   make up to, man of influence, manipulator, nag, narthex,
   open sesame, ordain, parliamentary agent, pass, persuade,
   pigeonhole, plead with, pocket, portal, portico, powers that be,
   press, pressure, pressure group, promote, propylaeum, pull strings,
   push, put in force, put through, railroad through, reception room,
   recommend, registered lobbyist, roll logs, sinister influence,
   soft-soap, special interests, special-interest group,
   special-interest lobby, stoa, sway, sweet-talk, table,
   take the floor, the Establishment, threshold, urge,
   very important person, vestibule, veto, wheedle, wheeler-dealer,
   wire-pull, wire-puller, work on, yield the floor