from The Century Dictionary and Cyclopedia
- noun Loss, through disease, of the ability to express musical sounds either vocally or in strumentally, to write musical notation (the power of ordinary writing being retained), or to appreciate musical sounds mentally.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- noun The inability to comprehend or respond to music.
Tone deafness -- or amusia – can be congenital, present from birth, or acquired following injury to the brain.
About four percent of the population has congenital amusia, a lifelong disability that prevents otherwise normal functioning individuals from developing basic musical skills.
The medical profession is still investigating the cause of amusia and it is possible that there is a genuine affliction that disables the ability to distinguish musical intonation, but in the vast majority of cases, even the most excruciating singer can be taught to sing in tune.
The word 'amusia' comes from a Greek word meaning 'want of harmony'.