Je ne sais quoi is an idiomatic expression from French which literally means, “I don’t know what.” In English, we use it when referring to something that is difficult to describe, often in a positive context.
For instance, if you see woman walk by, and you think she is attractive but you can’t quite pinpoint why, you might say, “She has that je ne sais quoi,” or “That girl has a certain je ne sais quoi.” In this example, it is used in the same sense as you might use either of the English phrases “a sense of allure” or “an air of mystery.”
A one-word English equivalent that would work in place of this phrase is ‘something.’ For instance, you could say, “She has that something about her” or “That girl has a certain something.” The phrase ‘something-or-other’ would probably also work in this context.
A couple of other one-word equivalents that might work, depending on the context, are: ‘intangible’ and ‘indescribable.’ Some lengthier examples you could use would be phrases like ‘impossible to describe,’ ‘defies description,’ and ‘I have no words.’
In a positive context, particularly referring to someone’s appearance or sense of confidence, one might also use words like ‘panache,’ ‘allure,’ and ‘pizzazz.’ You could also use the term ‘x-factor’ in many cases.
While there is no one-word exact equivalent to the French phrase, all of the above examples offer similar connotations that could be used to convey the same thing, depending on the context.