Which concept refers to the use of the same programming code across multiple applications?

The concept of re-usability is focused on the ability to use existing code to serve multiple purposes or applications without needing to rewrite it. This practice enhances efficiency in software development, as it allows developers to leverage previously created functions, classes, or modules in new projects, which saves time and effort. Re-usability is a fundamental principle in software engineering because it promotes maintainability and consistency across applications.

In contrast, scalability refers to a system's ability to handle growth or increased loads, while modularity involves structuring a program into discrete components or modules that can be developed or understood independently. Generality pertains to writing code that can work in a variety of situations or contexts, but does not specifically address the aspect of utilizing the same code across multiple applications in the manner that re-usability does. Thus, the concept that best captures the idea of using the same programming code multiple times in different applications is indeed re-usability.