The sensation of bodily equilibrium and changes in this sensation due to the acceleration or deceleration of the head (and therefore usually the whole body) and the rotation of the head; i.e. all movements that activate sensory cells in the balance organ.
In fact, there are three different "senses" that contribute to being able to "stay upright and in full balance"!
1/ The sense of balance, of course!
2/ The sense of sight and
3/ Muscle-joint sense.
You can helpfully manage to "keep your balance" if two of the above senses work. But "loses balance" if only one works.
The typical case is a heavily intoxicated person whose balance and muscle joint senses are blunted and who falls over when he closes his eyes or the light goes out in the room and it gets dark.