Ordinal number is a mathematical concept used to represent numbers in order or sequence. Ordinal numbers are expressed by adding the word "first" before integers, such as "first", "second" and "third". In addition, there are some established expressions, such as "first time, last time, first month, eldest son, youngest daughter". Ordinal number also has an important application in set theory, which is based on the concept of well-ordered set. In mathematics and related fields, such as computer science, the concept of order is everywhere. Ordinal theory is a branch of mathematics, which is an intuitive concept to study order by using binary relations (such as greater than and better than), and provides a formal framework for describing sentences such as this less than or this before.