noun
Noun for a fault, error, or blameworthy point, especially in set phrases such as 非を認める and 非がある.
彼は自分の非を素直に認めた。
He honestly admitted his own fault.
この件では、どちらに非があるのか分からない。
In this matter, I do not know which side is at fault.
noun
unfavorable situation; disadvantage; things going badly
Mostly literary or fixed-phrase use for a situation turning against someone; commonly seen in expressions like 形勢非なり.
形勢非なりと見て、彼らは撤退を決めた。
Seeing that the situation was turning against them, they decided to withdraw.
prefix
Prefix attached mainly to Sino-Japanese nouns or noun-like bases to mean 'not', 'non-', or 'outside the normal category', as in 非公開 and 非公式. Often has a formal or written tone.
この会議は非公開で行われる。
This meeting will be held behind closed doors.
非正規雇用で働く人が増えている。
The number of people working in non-regular employment is increasing.
A Sino-Japanese morpheme read ひ and written 非, conventionally associated with negation, wrongness, and 'non-' meanings. No detailed historical derivation is asserted here.