A person who isn't deaf, but nor are they hearing. Often a hard of hearing person will feel like they have a "foot on both sides of the fence." Many people make the incorrect assumption that a hard of hearing person can hear everything, only less loudly. Group situations are very difficult for hard of hearing people, so many will avoid them.
When you're speaking to a hard of hearing person, you should be facing them, speaking in a clear, even voice, and make sure that you're annunciating your words properly.