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]