What is the practice of improving internal code structure without changing its functionality called?

The practice of improving internal code structure while maintaining its existing functionality is known as refactoring. Refactoring involves reorganizing and optimizing existing code to make it more understandable and efficient, without altering what the code actually does. This process can enhance code readability, reduce complexity, and make future modifications easier, all of which contribute to better maintainability of software.

While quality assurance relates to ensuring that the software functions correctly, it does not focus specifically on code structure improvement. Scalability refers to a system's ability to handle growth in demand or workload, which is different from refactoring in terms of code structure. An API, or application programming interface, defines how different software components interact but is not directly related to modifying existing code for clarity or efficiency. Thus, refactoring stands out as the key concept in this context.