Correct spelling, explanation: acoustic entered English in the times of Middle English from a Medieval Latin form acousticus. However, the origin of the word can be tracked in Ancient Greek, in the word akoustikos, which meant of or for hearing. When it comes to the science of sound, the word acoustic was first used at the beginning of the 18th century.

Definition of acoustic:
1. adjective – something which is related to sound or hearing,
We still need to do some acoustic tests before the concert.
2. adjective – refers to the type of musical instrument,
I can play both – an acoustic guitar and an electrical one, but I prefer the former one.
3. noun – the way the structure of a building is related to the sound qualities of it,
The building has wonderful acoustics so various concerts often take place there.

Expressions with acoustic:
Popular expressions with acoustic include:
1. acoustic guitar – a type of guitar, which isn’t made louder by some electrical equipment; This version of the song with an acoustic guitar is even better than the original.
2. acoustic neuroma – a type of medical condition; The doctor diagnosed my father with acoustic neuroma.

Incorrect spelling, explanation: the word acoustic causes some troubles when it comes to spelling as many users of English spell it with a double c letter, which gives an incorrect form accoustic. It’s probably the consequence of the fact that many words in English with similar pronunciation indeed feature a double c letter, e.g. accuse. In the case of this word, however, the only correct spelling is acoustic.