严肃 (yán sù) is an adjective meaning 'solemn' or 'serious', describing a person’s demeanor, tone, expression, or atmosphere that conveys gravity, discipline, and lack of frivolity. The character 严 means 'strict' or 'rigorous', while 肃 means 'solemn' or 'austere'; together, they reinforce a sense of dignified seriousness—often associated with authority, formal settings, or moral weight. It commonly modifies nouns like 态度 (attitude), 表情 (expression), or 气氛 (atmosphere), and can also function as a predicate adjective.

This word carries a respectful, sometimes slightly stern connotation—it’s appropriate in professional, academic, or ceremonial contexts, but rarely used to describe casual or playful situations. Unlike the more neutral 严肃, words like 认真 (serious-minded) or 庄重 (dignified) have softer or more specific nuances. 严肃 implies both internal composure and external restraint, often suggesting adherence to principle or responsibility.