These words mean to enter a state of unconsciousness, impairment of awareness or extreme weakness. To faint is to lose consciousness suddenly because of a temporary deficiency of the blood supply to the brain. Fainting (for which the medical term is syncope) may be caused by emotional shock, pain, over-exertion or exposure to high temperatures. Swoon was once a synonym for faint , but is now obsolescent. Swoon appears widely in literature of the past, especially in the 19th century, where it often evokes the stock image of the delicate, vapourish lady of the Victorian period who habitually faints when under stress or in the grip of strong feelings: She swooned under the rain of his kisses; to swoon at the sight of blood.

Black out is used of aircraft pilots experiencing partial or complete loss of vision and sometimes of consciousness, caused by rapid changes in velocity during flight. By extension, black out has come to mean any temporary memory lapse, usually without fainting , especially one resulting from emotional tension or the consumption of large amounts of alcohol.

• The accused claimed that he had blacked out and did not remember strangling the murdered man; Jonathan was able to recall very little of what happened at the party because he black out early in the evening.

Pass out is an informal term for faint : to pass out from the heat; to pass out from fright. Unlike faint , pass out carries a suggestion of humour with little or no medical connotation, especially in its most popular use to describe falling into the stuporous sleep that overtakes some people after they have had too much to drink: a derelict who had passed out in a doorway; She was able to down one cocktail after another without passing out .

Collapse is a much stronger term than the other words and may indicate grave illness. To collapse may also involve fainting , but the word, from its literal meaning of giving way or caving in, emphasizes a complete and sudden loss of vital strength or health, which may be either brief or prolonged: those who collapse into bed after an exhausting day; He collapsed from overwork; to collapse from nervous strain; to collapse with a heart attack.