千秋 literally means 'a thousand autumns' and figuratively refers to a very long time, often used in expressions like 千秋万代 (for generations) or 千秋大业 (a great undertaking lasting millennia). It is also a formal and respectful way to offer birthday congratulations to an elderly person, similar to 'long life' wishes. The word carries a classical, elevated tone and is most common in written or ceremonial Chinese.