primary cause; main factor
Used in formal or analytical contexts to identify the most important reason or factor behind a situation, event, or problem.
The primary cause of the accident was the driver's carelessness.
Experts are analyzing the main factors behind the economic downturn.
The strong yen became the main cause of the export slump.
The plan was cancelled, mainly due to a lack of funds.
The main cause of the accident lies in the driver's carelessness.
要因 refers to any contributing factor in an analysis, whereas 主因 designates the single dominant factor.
主因 designates the main or primary cause, whereas 一因 indicates just one of several contributing causes, not necessarily the most important.
遠因 is an indirect, distant cause, while 主因 is the direct, principal cause of something.
Compound of 主 (しゅ, 'main') and 因 (いん, 'cause'). The reading is on'yomi, and the word is used in formal written Japanese.