A plague pit is the informal term used to refer to mass grave

Mass grave

A mass grave is a grave containing multiple number of human corpses, which may or may not be identified prior to burial. There is no strict definition of the minimum number of bodies required to constitute a mass grave, although the United Nations defines a mass grave as a burial site which...

s in which victims of the Black Death

Black Death

The Black Death was one of the most devastating pandemics in human history, peaking in Europe between 1348 and 1350. Of several competing theories, the dominant explanation for the Black Death is the plague theory, which attributes the outbreak to the bacterium Yersinia pestis. Thought to have...

 were buried. The term is most often used to describe pits located in Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

, but can be applied to any place where Bubonic plague

Bubonic plague

Plague is a deadly infectious disease that is caused by the enterobacteria Yersinia pestis, named after the French-Swiss bacteriologist Alexandre Yersin. Primarily carried by rodents and spread to humans via fleas, the disease is notorious throughout history, due to the unrivaled scale of death...

 victims were buried.

Origin

The plague which swept across Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 in the 14th century is estimated to have killed between one-third and two-thirds of Europe's population

Medieval demography

This article discusses human demography in Europe during the Middle Ages, including population trends and movements. Demographic changes helped to shape and define the Middle Ages...

. Disposal of the bodies of those who died presented huge problems for the authorities, and eventually the normal patterns of burial and funerary observance broke down.

Major plague outbreaks

Plague pits were used especially often during major plague outbreaks, such as the London epidemic of 1665

Great Plague of London

The Great Plague was a massive outbreak of disease in the Kingdom of England that killed an estimated 100,000 people, 20% of London's population. The disease is identified as bubonic plague, an infection by the bacterium Yersinia pestis, transmitted through a flea vector...

. During these times, graveyards rapidly filled, and such graves became available only to wealthy people. Parishes became strained; one example, the records of St. Bride on Fleet Street during the London 1665 plague, shows typical methods employed by the parishes.
In 1665, the total number of deaths in the parish rose to five and a half times their normal number, with 2,111 deaths overall and 1,427 attributed to plague.

Controversy

Some scientists have put forward the idea that the Black Death was not caused by yersina pestis as some have thought, and some evidence of this has been found in plague pits, where other diseases, such as anthrax

Anthrax

Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...

, have been discovered.