The type of hearing aid that is least visible is called an “invisible-in-the-canal” (IIC) hearing aid. It is custom-made to fit deep in the ear canal and is virtually invisible when worn.
Other types of hearing aids that are less visible include “completely-in-the-canal” (CIC) and “in-the-canal” (ITC) hearing aids, which are also custom-made to fit in the ear canal but are slightly larger than IIC hearing aids.