Miasma is a term that has been used in different contexts throughout history. Here are some of the main definitions and uses of the term:

• Medical theory: The miasma theory was an abandoned medical theory that held that diseases, such as cholera, chlamydia, or the Black Death, were caused by a miasma, a noxious form of "bad air", also known as night air. The theory held that epidemics were caused by environmental factors such as contaminated water, foul air, and poor hygienic conditions. Such infection was not passed between individuals but would affect individuals within the locale that gave rise to such vapors. It was identifiable by its foul smell.

• Greek mythology: In Greek mythology, miasma means “stain,” “defilement,” or “the stain of guilt”. It is usually translated as “pollution” in English, although there is no concept in English that precisely corresponds to miasma. Miasma is a god-sent disease that is caused by a murder that has not been atoned for (with proper purification rituals). A miasma can fall upon an entire city when one man in that city is guilty of a murder and has not atoned for it. A miasma can infect everyone on board a ship if one man on that ship is guilty of murder. Miasma can spread like a disease, and it seems to be the objectification of guilt. In other words, the guilt is understood as a kind of disease that can spread to everyone who comes in contact with the guilty person.

• General definition: Miasma is a poisonous vapor or mist believed to be made up of particles from decomposing material that could cause disease and could be identified by its foul smell. The miasma theory of disease originated in the Middle Ages and persisted for centuries. During the Great Plague of 1665, doctors wore masks filled with sweet-smelling flowers to keep out the poisonous miasmas. Because of the miasmas, they sanitized some buildings, required that night soil be removed from public proximity and had swamps drained to get rid of the bad smells. Although the miasma theory proved incorrect, it represented some recognition of the relation between dirtiness and disease. It encouraged cleanliness and paved the way for public health reform.

• Scientific definition: Miasma theory held that soil polluted with waste products of any kind gave off a miasma into the air, which caused many major infectious diseases of the day.

In summary, miasma can refer to a poisonous vapor or mist believed to be made up of particles from decomposing material that could cause disease and could be identified by its foul smell, a medical theory that held that diseases were caused by a noxious form of "bad air", a concept in Greek mythology that refers to the objectification of guilt, or a scientific theory that held that soil polluted with waste products of any kind gave off a miasma into the air, which caused many major infectious diseases of the day.