Lobbying is a form of advocacy, which lawfully attempts to directly influence legislators or government officials, such as regulatory agencies or judiciary. Lobbying involves direct, face-to-face contact and is carried out by various entities, including individuals acting as voters, constituents, or private citizens, corporations pursuing their business interests, nonprofits and NGOs through advocacy groups to achieve their missions, and legislators or government officials influencing each other in legislative affairs.

Lobbying or certain practices that share commonalities with lobbying are sometimes referred to as government relations, or government affairs and sometimes legislative relations, or legislative affairs. It is also an industry known by many of the aforementioned names, and has a near-complete overlap with the public affairs industry. Lobbyists may fall into different categories: amateur lobbyists, such as individual voters or voter blocs within an electoral district; professional lobbyists who engage in lobbying as a business or profession; and government relations support staff who work on behalf of professional lobbyists but may not actively engage in direct influencing or face-to-face meetings with targeted individuals to the extent required for registration as lobbyists, operating within the same professional circles as registered lobbyists.

Professional lobbyists are people whose business is trying to influence legislation, regulation, or other government decisions, actions, or policies on behalf of a group or individual who hires them. Nonprofit organizations whether as professional or amateur lobbyists can also lobby as an act of volunteering or as a small part of their normal job. Governments often define "lobbying" for legal purposes, and regulate organized group lobbying that has become influential.

In pre-modern political systems, royal courts provided incidental opportunities for gaining the ear of monarchs and their councilors.

That architectural sense of lobby is believed to originate from the medieval Latin lobia or lobium, which refers to a gallery, hall, or portico. This architectural sense was later adopted to describe the practice of advocating or debating in such spaces.

In a report carried by the BBC, an OED lexicographer has shown that "lobbying" finds its roots in the gathering of Members of Parliament and peers in the hallways ("lobbies") of the United Kingdom Houses of Parliament before and after parliamentary debates where members of the public can meet their representatives.

One story held that the term originated at the Willard Hotel in Washington, D.C., where it was supposedly used by President Ulysses S. Grant to describe the political advocates who frequented the hotel's lobby to access Grant—who was often there in the evenings to enjoy a cigar and brandy—and then tried to buy the president drinks in an attempt to influence his political decisions. Although the term may have gained more widespread currency in Washington, D.C., by virtue of this practice during the Grant Administration, the OED cites numerous documented uses of the word well before Grant's presidency, including use in Pennsylvania as early as 1808.

The term "lobbying" also appeared in print as early as 1820: