客气 (kèqi) is a social adjective (and verb) meaning 'polite, courteous', but in everyday Chinese it is most often used in the set phrases 别客气 ('don't stand on ceremony') and 不客气 ('you're welcome'). Unlike 礼貌 (lǐmào), which is a general term for politeness and manners, 客气 specifically describes the reserved, formally polite behavior you show to avoid offending someone, especially when declining offers or showing gratitude.