A lobby can be:

• An entryway or waiting area, such as a foyer, from the Latin word lobium , or vestibule.
• A generic term for groups involved in lobbying, such as the "petroleum lobby" or "environmental lobby", or as a verb such as "trying to lobby Parliament or Congress to change the law".

The former term led to the latter term, as those trying to change the laws often gather in the lobby of a capitol, to catch legislators as they come and go about their business.

(Ed. note: The first use of "lobbyist" has been attributed to a U.S. president, sometime around 1837. It seems impossible to find any info to confirm or refute this however. Can anyone else find anything on this?)