世 is a bound morpheme (b.f.) — it cannot stand alone as a word. It appears as the first or second character in many common compounds. Its core meanings trace back to 'generation' (30 years) and 'life', but it has expanded to cover 'era', 'epoch', and 'world'. Key compounds include 世界 (shìjiè, world), 世纪 (shìjì, century), 世代 (shìdài, generation), and 一生一世 (yīshēng yīshì, a whole lifetime).