一世 is a noun with several related senses. It can mean 'a lifetime' (一生), 'a generation' (一代), or 'the whole world' (全世界). The sense 'a moment' is less common and often appears in literary contexts. In classical Chinese, 一世 also referred to an era or epoch. Be careful: 一世 is not used as an adverb; the gloss 'momentarily' is likely a misattribution.