What is "ground distance" in trigonometry?

I am in Trigonometry, the book I am using seems to have said that the ground distance is the distance between a landing plane, and the runway (which would make it the hypotenuse), then another example makes it seem that the ground distance is actually the distance from the runway to the point directly under the plane (which would make it the opposite side), im sure one of these is wrong, which is it?