Zipcar counts as a disruptive innovation. Uber doesn’t. The latter is according to Clayton Christensen, Michael Raynor, and Rory McDonald in their recent HBR article “What is Disruptive Innovation?” The authors explain that disruption “describes a process whereby a smaller company with fewer resources is able to successfully challenge established incumbent businesses.” They also write that “disruptive innovations originate in low-end or new-market footholds.”