秋天 (qiū tiān) means 'autumn' or 'fall' — the third season of the year, occurring between summer and winter. The character 秋 (qiū) originally depicted grain ripening and symbolizes harvest and maturity; 天 (tiān) means 'sky' or 'heaven', but in seasonal compounds like this, it functions as a nominal suffix indicating a period or time — similar to how 春天 (chūn tiān, spring), 夏天 (xià tiān, summer), and 冬天 (dōng tiān, winter) are formed.

This word is used in both formal and everyday speech to refer to the season itself, its associated weather (cooling temperatures, falling leaves), cultural activities (harvest festivals, Mid-Autumn Festival), or poetic imagery. It commonly appears in time expressions (e.g., 去年秋天, 'last autumn'), descriptive phrases (e.g., 秋天的风, 'autumn wind'), and as a subject or object in sentences. Unlike English, Chinese does not use articles before 秋天, and it rarely requires measure words unless quantified (e.g., 一个秋天, 'one autumn').