Ground Clearance
The distance between the lowest point on the vehicle and level ground.
So we should measure it from the bottom of our tires
Toyota is stupid, they give two measurements for their vehicles. One that's "general", I think measured from the bottom of the axel tubing. The other is the "absolute", measured from the diff. For example, the Sequoia is quoted as 11.2 inches or something, but then toyota tells you it's 9.1 or something under the differential.
But then again, I'm sure the Hummer H1 is not 17 inches at the absolute lowest point. I think they measured it from the center between the two wheels. And since it has independent suspension, it's very high in the center.