Asonia

A person who is tone deaf lacks relative pitch, the ability to discriminate between musical notes. Being tone deaf is having difficulty or being unable to correctly hear relative differences between notes; however, in common usage, it refers to a person's inability to reproduce them accurately. The latter inability is most often caused by lack of musical training or music education and not actual tone deafness.

The ability of relative pitch, as with other musical abilities, appears to be inherent in healthy functional humans. The hearing impairment appears to be genetically influenced, though it can also result from brain damage. While someone who is unable to reproduce pitches because of a lack of musical training would not be considered tone deaf in a medical sense, the term might still be used to describe them casually. Someone who cannot reproduce pitches accurately, because of lack of training or tone deafness, is said to be unable to "carry a tune." Tone deafness affects ability to hear pitch changes produced by a musical instrument and/or the human voice. However, tone deaf people seem to be only disabled when it comes to music, and they can fully interpret the prosody or intonation of human speech. It has been observed that in societies with tonal languages such as Cantonese and Vietnamese, there are almost no tone deaf people. This might point to it being a learnt skill.

Tone deaf people often lack a sense of musical aesthetics, and much like a color blind person would not be apt to appreciate colorful visual art, some tone deaf people cannot appreciate music. Tone deafness is also associated with other musical-specific impairments such as inability to keep time with music (the lack of rhythm), or the inability to remember or even recognize a song. These disabilities can appear separately but some research shows that they are more likely to appear in tone deaf people. 

Tone deafness is also known variously as amusia, tune deafness, dysmelodia and dysmusia.