不法分子 is a formal, often official term for 'lawless elements' or 'criminals' — people who break the law. The word 分子 (fènzǐ) here means 'member' or 'element' (as in 积极分子 'activist'), not 'molecule'. It's commonly used in news reports, government announcements, and legal contexts to refer to individuals or groups engaged in illegal activities. Unlike the more general 坏人 (bad person), 不法分子 carries a stronger sense of being a disruptive or dangerous element in society.