What is a static variable?

A static variable is one that retains its value between function calls, which aligns with the definition provided in your response. When a static variable is declared within a function, it maintains its state even after the function has completed execution. This means that the next time the function is called, the static variable will still hold the value it had before the most recent call. This is particularly useful for keeping track of information that needs to persist across multiple invocations of a function, such as counting how many times the function has been called.

In contrast, variables that are limited to local scope (as referred to in another option) are not stored persistently beyond the function's execution and will be initialized each time the function runs. Immutable variables refer to those which cannot be modified after their declaration; this concept is different from the persistence attribute of static variables. Furthermore, private variables are associated with the encapsulation in object-oriented programming and restrict access to the class they belong to, which does not relate to the concept of persistence between function calls.

Overall, understanding static variables is crucial for managing stateful information across function invocations in programming, making them an essential concept in C and many other programming languages.