the ear: the organ of hearing, and by extension the readiness to take in and heed what is said
This is the ordinary word for the ear, the part of the body that hears. The Gospels use it plainly for the physical organ, from the ears Jesus touched when he healed a deaf man to the servant's ear cut off in the garden. When Paul asks whether the ear can resign from the body because it is not an eye, he means the literal organ too.
Most often, though, the New Testament uses the ear for something the body part only pictures: the inner willingness to listen and obey. Jesus' repeated call, "whoever has ears to hear, let them hear," and the same refrain running through the letters to the seven churches, is not about physical hearing at all. Everyone in the crowd could hear the sound; the question is whether they will truly receive it. The reverse image is just as common: ears that are heavy, dull, or (in the old prophetic phrase) uncircumcised belong to people who can hear the words but refuse to let them land.
A few uses turn on the ear as the place where a message arrives or is dropped quietly. To speak "in the ear" is to whisper privately, and to say a thing is fulfilled "in your ears" is to say it in your hearing. When Scripture pictures cries reaching the ears of the Lord, or his ears being open to prayer, it borrows the same everyday image to say that God attends and responds.