I don't know if there is as far as "Law" goes. There should be though. Most people can understand how someone can be "Legally blind" but still have some ability to see (Things close, far away, see shadows and light, etc...). But most people can't understand a deaf person who has some hearing ability left. When they think "deaf" they think "No sound at all" and that isn't accurate.
Personally I think a guideline to consider someone "Legally deaf" would be is their hearing loss prevents them from being able to hear spoken communication by sound alone. That's when I considered my own hearing no longer being "Hard of hearing" but deaf.
For a few years after my hearing dropped from profound hearing loss to deafness. I could still hear some phones ring but I could not hear anything on the phone (Could not hear anyone's voice over the phone). I could hear some peoples voices if it was loud and low pitched enough but I could not hear what the voices where saying. I could hear a dog bark and a car horn but they where faint sounds that I usually couldn't identify without seeing them.
This is when I considered myself "Legally deaf" and I feel that if there is a legal term for it then this is where that line would be drawn. There is a line between someone with "very bad eye site" and a person who is "legally blind". There are differences between them in both our legal system and in some cases other areas such as insurance. Deafness should have a similar distinction.
Ron Jaxon