If "fair" means "just" or "impartial," the abstract noun is justice or impartiality.
If "fair" means "attractive" or "pleasing," the abstract noun is beauty or charm.
If "fair" refers to a specific event, the abstract noun is fairness or festival.
For example:
* "The judge ensured justice was served." (meaning "fairness")
* "She was known for her beauty and grace." (meaning "attractive")
* "The local fair was a huge success." (meaning "event")