A hearing aid is a device which people with hearing difficulties wear in their ear to enable them to hear better.
hearing aid in British English
noun
a device for assisting the hearing of partially deaf people, typically consisting of a small battery-powered electronic amplifier with microphone and earphone, worn in or behind the ear
hearing aid in American English
a small, battery-powered electronic device that amplifies sound waves, worn in or near the ear of a person who is partially deaf