"An egg" is the correct Engpsh term for "一只鸡蛋".

Its origins come from Middle Engpsh, where "egg" was used to describe the round, oval shaped object that is produced by birds such as chickens and other poultry. The word has been in use in Engpsh since the Old Engpsh period, and has been adopted in many other languages as well.

Here are some Engpsh sentences using "egg":

I would pke an egg for breakfast.
(我想吃一个鸡蛋早餐)
The eggs were well-cooked.
(鸡蛋煮得很好)
He was cracking an egg into the pan.
(他正在把鸡蛋敲进锅里)