Here's what might be happening:
* Misunderstanding of "mental": Perhaps the speaker meant to say "abstract noun." Abstract nouns refer to concepts, qualities, ideas, and feelings, which are not physical objects. Examples include: *love*, *justice*, *peace*, *happiness*.
* Specific context: It's possible that "mental noun" has a very specific meaning within a particular field of study or a particular group's jargon.
To understand what is meant by "mental noun," you need more context.
Please provide more information:
* Where did you hear the term?
* What was the context?
* Can you give an example of a "mental noun" in use?
This will help clarify what the speaker actually meant.