hearing aid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small electronic apparatus that amplifies sound and is worn in or behind the ear to compensate for impaired hearing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device used to help the hard of hearing to hear sounds.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an electronic device that amplifies sound and is worn to compensate for poor hearing
• noun a conical acoustic device formerly used to direct sound to the ear of a hearing-impaired person