What does the term 'nested' refer to in programming?

The term 'nested' in programming most commonly refers to code that is contained within another function. When a function is defined within another function, it is considered a nested function. This structure allows the inner function to access variables and parameters from the outer function's scope, which can be particularly useful for encapsulating functionality and managing variable visibility.

Nested functions can help improve modularity and maintainability of the code by allowing complex operations to be broken down into simpler, reusable components. Additionally, they can facilitate the creation of closures—functions that remember the environment in which they were created. This is a powerful feature in many programming languages, enabling advanced patterns such as callbacks and asynchronous programming.

While the other choices touch on relevant programming concepts, they do not specifically capture the essence of what 'nested' means as accurately as the notion of having one block of code contained within another. For instance, sequential operations describe the order in which functions execute rather than their structural relationship. Hierarchical data structures pertain to organizing information rather than the relationship between functions or blocks of code. Lastly, defining variables within other variables isn’t a standard definition of 'nested' in programming, as this wording might imply a different concept, like variable scope or dependency.