In physics, "At mean n" is not a standard phrase or notation. It's likely a misunderstanding or a misinterpretation of a specific concept.

To help me understand what you're looking for, could you please provide more context?

For example:

* Where did you encounter this phrase? Was it in a textbook, a lecture, or an online article?

* What was the surrounding text or the problem you were trying to solve?

* What are you trying to understand about this phrase?

With more information, I can help you understand the correct meaning.