A fear of heights is called acrophobia. Acrophobia can range from fear when on the top floor of a tall building, to fear of standing on a chair. People with acrophobia feel a sense of panic when they're at a certain height and often become unable to trust their sense of balance. Other symptoms can include shaking, dizziness and nausea. That's a pretty extreme reaction to standing on a chair!

More Info: en.wikipedia.org