Ground clearance is a measurement of the distance between the ground and the undercarriage of a vehicle. It is usually shown as the ground clearance in the center, since on most vehicles upon which it matters the differential is the lowest point, a typical characteristic of vehicles with a fixed axle.
Some vehicles have greater ground clearance across the entire span of the machine because they use various types of independent suspension. Perhaps the most notable member of this class is the Hummer/HMMVR/Hum-Vee which has 18 inches/46 centimeters of ground clearance until one is just next to the wheel.
Ground clearance is often remarked on when it comes to lowered sports cars as well, as the process frequently causes them to scrape when entering or exiting driveways and other such generally innocuous hazards. Adding a "skirt package" (or ground effects) to a car reduces the ground clearance. It is often done purely for looks, but doing this (especially when coupled with the act of lowering the car through the use of different springs or spring limiters) also reduces aerodynamic lift.