Real Dictionary

lobby
countable noun: A lobby is a group of people who represent a particular organization or campaign, and try to persuade a government or council to help or support them.
countable noun: In a hotel or other large building, the lobby is the area near the entrance that usually has corridors and staircases leading off it.
verb: If you lobby someone such as a member of a government or council, you try to persuade them that a particular law should be changed or that a particular thing should be done.

lobby in American English
a hall or large anteroom, as a waiting room or vestibule of an apartment house, hotel, theater, etc.
intransitive verb: to solicit or try to influence the votes of members of a legislative body
noun: an entrance hall, corridor, or vestibule, as in a public building, often serving as an anteroom; foyer
transitive verb: to try to influence the actions of (public officials, esp. legislators)

lobby in British English
noun: a room or corridor used as an entrance hall, vestibule, etc
verb: to attempt to influence (legislators, etc) in the formulation of policy

lobby in Hospitality
noun: In a hotel or other large building, the lobby is the area near the entrance that usually has corridors and staircases leading off it.