Meaning:
“白” means “white,” and “月光” means “moonlight.” Quoted from late Chinese author Eileen Chang’s novel “Red Rose, White Rose,” this term refers to a spotless woman a man cherishes in his memory whom he loved but who is no longer with him. The term caught on with the popularity of a TV series by the name of “Yanxi Palace,” in which the role of a deceased queen is called by netizens “the emperor’s white moonlight.”
Example:
A: 你哥都奔四的人了，怎么还不结婚？
Nǐ gē dōu bēnsì de rén le，zěnme hái bù jiéhūn？
Your elder brother is almost 40. Why is he still single?
B: 他大学谈了一次恋爱，那个女生成了他念念不忘的白月光。
Tā dàxué tán le yīcì liàn’ài，nàge nǚshēng chéng le tā niànniàn búwàng de báiyuèguāng。
He had a romance back in university, and that girl became his white moonlight (the cherished love of his life).