will-o'-the-wisp
noun [ C usually singular ] uk/ˌwɪl.ə.ðəˈwɪsp/ us/ˌwɪl.ə.ðəˈwɪsp/
虚幻的目标;实现不了的事
something that is impossible to get or achieve
Full employment is the will-o'-the-wisp that politicians have been chasing for decades. 全民就业是政治家们几十年来一直在追求的一个幻想。