「耳 (みみ)」 means "ear." This word refers to the organ on the side of the head, used for hearing sounds. You use it when talking about listening to music, hearing something, or describing someone's ears.

音(おと)を聞(き)く体(からだ)の器官(きかん)です。耳(みみ)がよく聞(き)こえます。