A ban is a formal or informal prohibition^[1] of something. Bans are formed for the prohibition of activities within a certain political territory. Some see this as a negative act (equating it to a form of censorship or discrimination) and others see it as maintaining the "status quo". Some bans in commerce are referred to as embargoes. Ban is also used as a verb similar in meaning to "to prohibit".

In current English usage, ban is mostly synonymous with prohibition. Historically, Old English (ge)bann is a derivation from the verb bannan "to summon, command, proclaim" from an earlier Common Germanic *bannan "to command, forbid, banish, curse". The modern sense "to prohibit" is influenced by the cognate Old Norse banna "to curse, to prohibit" and also from Old French ban, ultimately a loan from Old Frankish, meaning "outlawry, banishment".^[1]

The Indo-European etymology of the Germanic term is from a root *bha- meaning "to speak". Its original meaning was magical, referring to utterances that carried a power to curse.

In many countries political parties or groups are banned. Parties may be banned for many reasons, including extremism and anti-democratic ideologies,^[2] on ethnic or religious grounds,^[3] and sometimes simply because the group opposes government policies, with the ban sometimes alleging wrongdoing as the cause.^[4] Germany, for instance, has a long history behind its modern practice of banning political parties. The Nazi Party was banned in 1923; after the Nazi Party came into power in 1933 opposing parties such as the Social Democrats (SPD) and Communist Party of Germany (KPD) were banned, the Nazi Party was again banned and the ban on other parties lifted after the Nazi defeat in 1945, and the Communist Party was again banned from 1956 to 1968.

There have been many bans on marriages, and sometimes other sexual liaisons, between people of different ethnic background or religion, for example between non-Jews and Jews in Nazi Germany, people classified as "white" and non-whites in apartheid South Africa, etc.

For much of the 1800s and 1900s there were bans on marriage between people of different races (interracial marriage) in many of the United States. However, the ban on interracial marriage was overturned by the Supreme Court of the United States in 1967 in the landmark civil rights case Loving vs. Virginia, in which the Court ruled Virginia's miscegenation law an unconstitutional violation of the fundamental right to marriage. Historically child marriage was common, but is now banned in many countries.

The Imperial ban was a form of outlawry in the Holy Roman Empire. At different times, it could be declared by the Holy Roman Emperor, by courts including the League of the Holy Court (German: Vehmgericht, pronounced [féːmgəʀɪχt]) and the Reichskammergericht, or by the Imperial Diet. People under Imperial ban lost all their rights and possessions, and anyone had the right to rob, injure or kill such persons without legal consequences. The Imperial ban automatically followed the excommunication of a person, and extended to anyone offering help to a person under the imperial ban.

During the apartheid régime in South Africa, the National Party government issued banning orders to individuals seen to be threats to its power — often black politicians or organizations — these banning orders acted as suppression orders. Individuals banned by the Suppression of Communism Act, 1950 (which effectively defined "Communism" as opposition to the government) could not communicate with more than one person at any time unless at home (thus preventing them from engaging in political activities), travel to outside a specific magisterial district without government approval, or leave the country.

Existing publications may be banned, and new publications prohibited from discussing certain topics or taking certain viewpoints. Censorship is the enacting and enforcing of such bans.

Bans in various jurisdictions on possession of some weapons, smoking, and narcotic drugs are enacted to exert control over the general public.