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The ratio of wheel travel to spring (or damper) travel. A motion ratio of 1.5:1 means the wheel moves 1.5 inches for every 1 inch of spring compression, which affects the effective wheel rate felt at the tire.
Carroll Smith illustrates the math in Tune to Win with a clear worked example: "Motion Ratio = Wheel Travel divided by Spring Travel. Wheel Rate = Spring Rate divided by (Ratio) squared. For a 400 lb/in spring with a motion ratio of 1.5: Wheel Rate = 400 / (1.5 squared) = 178 lb/in." He shows that with an inboard suspension layout where the motion ratio increases to 2.2, "for 178 lb/in wheel rate, the spring rate required is 861 lb/in" and conversely "for a 400 lb spring, the wheel rate = 400 / (2.2 squared) = 83 lb/in." The squared relationship is critical — small changes in motion ratio have a large effect on wheel rate. This is why simply comparing spring rates between different cars is misleading without knowing the motion ratio of each suspension design.