The withers is the highest point on an animal's back, on the ridge between its shoulder blades. The height of an animal is measured from the ground to the withers because that is a fairly stable and predictable height when the animal is standing still. Unlike humans, whose heads are in a fairly fixed position when standing upright and looking forward, animals can move their heads easily into a wide range of positions when standing still, so it makes a poor location for measuring the animal's height.
This term is used primarily for four-legged domestic animals, such as dogs and horses.
