It's when you're just sitting there watching TV, doing nothing, your hands are in your lap, and then your hand is shaking. Essential tremor is much more common. It's when you pick up a pen, go to write, and your hand is shaking.
People with a head tremor most often have essential tremor--that's not a feature of Parkinson's disease--but people with a chin tremor actually have Parkinson's and not essential tremor. There are all these little things that go along with trying to differentiate, but unless you see people with those conditions, it's.... In the medical community, a lot of physicians will get it wrong.