可将X翻译成“某”：

There was a sentence about X that I would have crossed out. 有个关于某内容的句子被我给删掉了。

又如：

Let us say Mr X gives a property to his son. 比如说，某先生把一份财产给了他儿子。

如果与Y并用，也可翻译成“甲”“乙”。如：

The children on trial were referred to as child X and child Y. 受试儿童被称作儿童甲和儿童乙。

如果是表示数字，则可直接用x表示。如：

Let x have the value 25. 设 x 的值为 25。

Make X greater than or equal to zero. 假设 X 大于等于零。