One of the 四大美女, the Four Beauties of ancient China. In legend King 勾践 of Yue sent her to the King of Wu so that Yue could take revenge. Her name means any beauty, as in 情人眼里出西施.
Beauty is in the eye of the beholder. (Lit. A lover's eyes produce Xi Shi.)
Tips
culture
Her name means 'Shi of the west village'. A plain neighbour, 东施, copied the way she frowned with a hand on her chest and only looked worse, which gave the idiom 东施效颦 for clumsy imitation.
culture
苏轼 compared Hangzhou's West Lake to her: 欲把西湖比西子，淡妆浓抹总相宜, 'liken West Lake to Xi Shi: light make-up or heavy, she always looks right'. So the lake is also called 西子湖.
