Audiology is the study of the hearing process and hearing disorders, usually with an emphasis on speech perception.
Hearing loss is decreased auditory perceptive ability. Such loss may be temporary, chronic or permanent, and may occur in various ways: through occupational noise exposure; through aging (presbycusis); or through exposure to modern urban noise.
Hearing ailments are dealt with by a branch of medicine called otology, and are largely the result of problems in the middle and inner ear.
Hearing loss, which is always acquired, should not be confused with deafness (anacusis), which refers to the complete inability to hear and is often congenital.
(Source - adapted and summarised from Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)