Translation guide
To become unconscious, either temporarily or for a longer period. In Japanese, the most common and natural way to express this is 気を失う (ki o ushinau), literally 'lose one's spirit/consciousness'. Other expressions exist for fainting, passing out, or losing awareness, with nuances of formality and cause.
To become unconscious, typically temporarily, due to shock, injury, or other causes.
The most common and neutral way to say 'lose consciousness'. Can be used for fainting, passing out from shock, or being knocked out.
彼は事故で気を失った。
He lost consciousness in the accident.
あまりの痛さに気を失いそうになった。
I almost lost consciousness from the pain.
A more formal or medical term for fainting or losing consciousness. Often used in written or clinical contexts.
患者は診察中に失神した。
The patient lost consciousness during the examination.
Literally 'lose awareness/consciousness'. Slightly more formal than 気を失う, and can imply a deeper or longer loss of consciousness.
彼は頭を強く打って意識を失った。
He hit his head hard and lost consciousness.
To faint or pass out, often from shock, fear, or pain. More colloquial than 失神する.
注射を見て気絶しそうになった。
I almost passed out when I saw the needle.
To suddenly lose consciousness and fall, often due to heat, shock, or low blood pressure.
Literally 'to fall down', but often used to mean collapsing or fainting. Context makes it clear if it's due to loss of consciousness.
暑さで倒れてしまった。
I passed out from the heat.
彼女は貧血で倒れた。
She fainted from anemia.
A somewhat literary or old-fashioned term for fainting or collapsing. Not commonly used in everyday speech.
その知らせを聞いて卒倒しそうになった。
I almost fainted when I heard the news.
To lose awareness of one's surroundings, often temporarily, e.g., from alcohol or a blow.
Colloquial expression meaning 'to black out' or 'lose awareness'. Often used for alcohol-induced blackouts or momentary lapses.
飲みすぎて意識が飛んだ。
I drank too much and blacked out.
殴られて一瞬意識が飛んだ。
I got punched and blacked out for a moment.
Specifically means 'to lose memory' of a period, often due to alcohol or trauma. Not exactly loss of consciousness, but related.
昨夜のことは記憶が飛んでいる。
I have no memory of last night.
気を失う is the most common and versatile. 意識を失う is more formal and often implies a deeper or longer unconscious state. 失神する is a medical/formal term for fainting. In casual conversation, 気絶する or 倒れる are often used for fainting.
無意識 (muishiki) means 'unconscious' in the psychological sense (subconscious), not the medical sense. Saying 無意識になる would mean 'become subconscious', which is incorrect for losing consciousness.