Reusability refers to the design philosophy and engineering capability of recovering and reflying launch vehicle hardware. SpaceX demonstrated this at scale with the Falcon 9 first stage, which has been landed and reflown over 300 times collectively, reducing the effective cost per launch by amortizing hardware across many flights. The Falcon 9 booster has achieved individual reuse records exceeding 20 flights.
The next frontier is full reusability, where both the first and second stages are recovered. SpaceX's Starship, Stoke Space's Nova, and Rocket Lab's Neutron all target some degree of upper-stage reuse. Full reusability could reduce launch costs by another order of magnitude, potentially bringing the cost to orbit below $100 per kilogram and enabling entirely new categories of space activity.