As a catch-all term for multiple forms of hearing loss, the actual meaning of the phrase hearing impairment depends largely on the context, as noted by WebMD.
The Individuals with Disabilities Act (IDEA) defines hearing impairment as "an impairment in hearing, whether permanent or fluctuating, that adversely affects a child’s educational performance but that is not included under the definition of deafness." Deafness, meanwhile, is defined as "a hearing impairment that is so severe that the child is impaired in processing linguistic information through hearing, with or without amplification, that adversely affects a child’s educational performance."
Hearing impairment is also commonly used by medical professionals to describe hearing loss in adults. Interestingly, the Social Security Administration's Blue Book does not directly reference hearing impairment, and instead uses hearing loss