1. the organ of hearing and balance. In humans and other mammals, the ear is divided into external, middle, and inner sections. The pinna of the external ear collects sounds that are then funneled through the external auditory meatus to the tympanic membrane. The sounds are vibrations of air molecules that cause the tympanic membrane to vibrate, which in turn vibrates the ossicles, three tiny bones in the middle ear. The motion of the last of these bones produces pressure waves in the fluid-filled cochlea of the inner ear. The motion of the fluid in the cochlea is converted by specialized receptors called hair cells into neural signals that are sent to the brain by the auditory nerve.

2. the projecting part of the external ear. See auditory system.