Real Dictionary

earphone
Earphones are a small piece of equipment which you wear over or inside your ears so that you can listen to music, the radio, or your phone without anyone else hearing.
countable noun: An earphone is the part of a telephone receiver or other device that you hold up to your ear or put into your ear.

earphone in American English
a receiver for radio, telephone, a hearing aid, etc., either held to the ear or put into the ear
noun: a sound receiver that fits in or over the ear, as of a radio or telephone

earphone in British English
noun: a device for converting electric currents into sound waves, held close to or inserted into the ear

earphone in Electrical Engineering
noun: Earphones are devices that are held close to or inserted into the ear, that convert electric currents into sound waves.