1 [C,U] (plural X's;x's /ˈeksɪz ||; ˈɛksɪz/) the twenty-fourth letter of the English alphabet 英语字母表的第二十四个字母: ‘Xylophone’ begins with (an) ‘X’. xylophone以字母X开头。

2 [U] (mathematics数学) used to represent a number whose value is not mentioned （用于代表未知数或变量）: The equation is impossible for any value of x greater than 2. 如果未知数x大于2，这个方程式就不能成立。

3 [U] a person, a number, an influence, etc that is not known or not named 未知的人、数及影响等: Let's suppose X knows what Y is doing. 我们假设X知道Y在做什么。 ☞The symbol X represents the number 10 in Roman numerals and it is also used by teachers to show that an answer is wrong. At the end of a letter it represents a kiss Lots of love, Mary XX. X代表罗马数字10。教师还用X表示答案错误。X在信末代表亲吻：Lots of love, Mary XX.献上我的爱，玛丽（吻你、吻你）