'Darling' is a term of endearment.

“亲爱的”是一种昵称。

《牛津词典》

No term of endearment crossed their lips.

他们彼此间没有说过爱慕的话。

《柯林斯英汉双解大词典》

English speakers borrowed the idea, and sweet + heart has been a term of endearment -particularly for romantic love -since the Middle Ages.

英语语言者借用了这一思想，于是从中世纪开始，sweet + heart就成为了一个表达爱意的词，尤其是浪漫的爱情。

youdao

The "lucky country" was meant as a dig rather than a term of endearment.

BBC: Essential reading for new arrivals in Australia

To her every address she appended a term of endearment: sweetness, monkey, pie-pie.

NEWYORKER: Soldier��s Joy

Indeed, the system is very "Tico, " Costa Ricans' term of endearment for themselves.

WSJ: Costa Rica Addresses Its Lack of Street Names