Any alteration of hearing capacity. Hearing impairment can be of various degrees, including mild, moderate, severe, profound, or total. The degree of impairment typically is categorized by the loss of hearing sensitivity, that is, how loud sounds must be for a listener to hear them. The degree of impairment can refer either to the loss of hearing sensitivity for individual pitches of sounds for each ear separately or to an overall loss of hearing sensitivity for both ears. Hearing impairment is further categorized as unilateral if present in only one ear and as bilateral if present in both ears.