This is the 577th most frequent Persian word.
دومین
"دومین" means "second."
Here, "دومین" specifies an ordinal number to signify the sequence of occurrences, indicating the second instance of an action.
این دومین بار است که به این رستوران میروم.
This is the second time I am going to this restaurant.
In this sentence, "دومین" is used to denote the ordinal position of a student among winners, indicating sequence.
او دومین دانشآموز از کلاس است که در مسابقه برنده شده.
He is the second student from the class who has won the competition.
In this context, "دومین" identifies the position of a part (story) within a whole (book), showing order.