世纪 (shì jì) literally means 'generation era' — 世 (shì) refers to a generation or age, and 纪 (jì) means 'era', 'epoch', or 'record'. Together, they form the standard term for a period of 100 years, equivalent to the English word 'century'. It is used in historical, chronological, and formal contexts — for example, when referring to events in the 20th century or describing long-term trends across centuries.

The word is always used with a numeral or ordinal prefix (e.g., 二十世纪, 'twentieth century') and never stands alone without modification. Unlike English, Chinese does not use 'century' as a countable noun in phrases like 'a century ago'; instead, it relies on time expressions like 一个世纪前 (yī gè shìjì qián). 世纪 is neutral in register but sounds more formal than colloquial alternatives like 百年 (bǎi nián, 'hundred years'), which is less precise chronologically.