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A rubber or urethane component that limits the maximum compression travel of the suspension. Modern bump stops are progressive — they resist more the further they are compressed — and are used as a tuning tool, not just a safety limit.
Staniforth's Car Suspension and Handling explains the fundamental need: "Suspension movement has to be limited to prevent metal-to-metal contact at the maximum bump limit of available wheel travel and springs becoming unseated at the corresponding rebound limit. The greater the movement that can be allowed, the better the ride can usually be." Halderman's Automotive Steering, Suspension, and Alignment provides the material definition: "Bump stop — a rubber or urethane stop to limit upward suspension travel. Also called a strikeout bumper, suspension bumper, or compression bumper." In racing applications, progressive-rate bump stops are specifically selected to add a rising spring rate at the end of travel, effectively creating a dual-rate suspension. Cars running very low ride heights deliberately use the bump stops as part of the spring package, with the base springs chosen to be softer knowing the bump stops will add rate under high-load cornering.