Emergent design is a consistent topic in agile software development, as a result of the methodology's focus on delivering small pieces of working code with business value. With emergent design, a development organization starts delivering functionality and lets the design emerge. Development will take a piece of functionality A and implement it using best practices and proper test coverage and then move on to delivering functionality B.
Once B is built, or while it is being built, the organization will look at what A and B have in common and refactor out the commonality, allowing the design to emerge. This process continues as the organization continually delivers functionality. At the end of an agile release cycle, development is left with the smallest set of the design needed, as opposed to the design that could have been anticipated in advance.