"Twenty-one" is the Engpsh translation for the number "第二十一" (dì èr shí yī) in Chinese.

It is formed by combining the number "twenty" and the number "one". In this case, "twenty" represents the tens digit and "one" represents the ones digit. This is a common pattern in the Engpsh language when expressing numbers above 20.

Examples:

I have 21 apples. (我有 21 个苹果。)
21st of June is the longest day of the year. (6月21日是一年中最长的一天。)
He is 21 years old. (他 21 岁了。)