Ān dìng (安定) is an adjective meaning 'stable' or 'peaceful', often describing a state of calm, security, or lack of disturbance. The character 安 means 'safe' or 'at ease', while 定 means 'to settle' or 'to fix firmly'. Together, they convey the idea of being firmly settled and free from anxiety, chaos, or volatility. It’s commonly used to describe physical conditions (e.g., blood pressure), mental states (e.g., mood), social environments (e.g., society), or even mechanical systems (e.g., equipment operation).

This word carries a formal and positive connotation and appears frequently in medical, governmental, and psychological contexts. While it can function attributively before nouns (e.g., 安定的生活), it’s also used predicatively after verbs like 是 or 得 (e.g., 情绪很安定). Unlike colloquial terms like 平静 (calm), 安定 emphasizes durability and reliability of stability—not just momentary quiet.