? About IS

What does X-Stop (2-stop, 3-stop, etc) IS mean?

I understand basic principles behind IS, but I don't understand using the metric "stop" to further define or place measurement on the Image Stabilization technology.

Is stop in any way related to a stop of light?

Yep, same term as stop of light. Just means that the IS has the potential to allow you to shoot at a shutter speed x number of stops slower than without IS.

Yes. It is a stop of light.

So if you shoot something without IS and get it stable at 1/200th of a second.
With a 4stop IS you can get 4 stops slower shutter. So you could get a 25th of a second exposure with the same results, if you move your hands the same
With a 3stops IS you will can get the same result at one 50th of a second with the example above.

With a 200mm lens you need about 200th (I think) of a second or faster if you don't want to see any blur (caused by shakey hands and body) in the image. With a good IS you can go to a lower shutterspeed. And if you can drop your shutterspeed you can shoot with a lower ISO and don't have to use the lens wide open as much, if you want too.

How does Canon (or other brands) measure the effectiveness (# of stops) of their IS technology?

Also, how many stops is the IS in the new 70-200 IS II?

Normally at 200mm the slowest shutter speed you should use is 1/200.

Canon claims 4 stops worth of Image Stabilization on the 70-200 IS II so you can expect to shoot at 4 stops slower shutter speed and still get sharp results.

4 stops slower than 1/200 would be 1/30 (1/200 > 1/100 > 1/50 > 1/25)