noun
non-prosecution; non-indictment
Legal term for a prosecutor's decision not to bring charges. Common in news reports and legal contexts.
検察は証拠不十分で不起訴とした。
The prosecution decided not to indict due to insufficient evidence.
不起訴処分に不服がある場合は、検察審査会に申し立てること
If you are dissatisfied with a non-prosecution decision, you can file a complaint with the prosecution review commission.
不起訴処分 is the full phrase for 'decision not to prosecute', often used interchangeably with 不起訴 in news.
Compound of 不 (non-) and 起訴 (prosecution, indictment). The term is a direct legal borrowing from Chinese-character-based legal vocabulary.