It is an anthropomorphic title.

Tianxier is an anthropomorphic term, usually used to describe the endearment between lovers or close relationships, and to describe sweetness. This word also means "darling" or "darling", which means that when you think of the other person, you will feel beautiful in your heart, and you will be completely happy.

Each couple has their own unique nicknames. These nicknames are usually based on each other's personality, hobbies, habits, etc., and are unique names for each other.