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A measure of how stiff a spring is, expressed in pounds per inch or Newtons per millimeter. Higher spring rates reduce body roll and weight transfer speed, but can make the car less forgiving over bumps.
Bentley defines the broader concept in Ultimate Speed Secrets: "Roll stiffness is the total amount of resistance to the car leaning or rolling provided by the springs and anti-roll bars. This is measured in pounds per inch of spring travel at the wheel. The distribution of the vehicle's roll stiffness between the front and rear suspension is called the roll stiffness distribution." Haney's The Racing and High-Performance Tire provides a worked example: with "a 300 lb/in rate at the front and 200 at the rear," adjustments to "percent-front weight transfer" directly change handling balance. He shows that going softer at both ends while maintaining the front-to-rear ratio lowers total roll stiffness without changing balance — the spring rate ratio between front and rear axles is the primary tuning lever for understeer/oversteer.