Vestibular refers to your sense of balance. Your sense organs that tells you which way is up and which ways your head is accelerating is the semicircular canals. They are closely related to your hearing.
The semi-circular canals are three tiny circular tubes at right angles to each other which contain fluid and can sense when that fluid is moving in each of the three dimensions that make up our 3D space.
There are two sets of semi-circular canals; one set adjacent to each cochlea.
Nerve bundles coming from the semi-circular canals join together with your auditory nerve and carry balance signals to your brain.
Many people with hearing loss also have some degree of balance difficulties, since the vestibular (or balance) system and the auditory (or hearing) systems are so closely related.
Many audiologists also have specialized training and equipment to help diagnose and treat people with balance problems.