What defines a global variable in C++?

A global variable in C++ is a variable that is declared outside of all functions, typically at the top of the source file. This positioning gives it a scope that extends throughout the entire program, allowing it to be accessed and modified from any function within that program. This characteristic is what fundamentally defines a global variable, and it plays a significant role in managing data that needs to be shared across different parts of a program.

In contrast, a variable declared inside a function (which would be local to that function) can only be accessed within that function, illustrating the limitations of local scope. Variables declared inside loops are also local and suffer from the same restriction. Furthermore, a compilation unit restricts visibility to only that specific unit, which may include a single file or module. This limitation further distinguishes it from global variables that are accessible program-wide. Therefore, the correct understanding of a global variable's definition is that it can indeed be accessed from any part of the program.