A scale-up is a business with a strong validated product/model focused on rapid growth. Typically, a business is considered scale-up after the startup stage, where they found a working product or model to scale.
Uber was a startup. They started as a limo service that people could order through an app. After testing different markets, they found and validated the idea of ordering a ride using the app. At that stage, they became a scale-up; they had a great working idea and focused on aggressive growth.