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A global variable is a variable declared outside any function, class, or module, and is accessible from any part of the program. It is typically declared at the beginning of a code file. Global variables can be used and modified by any function or module in the program, making them useful in cases where multiple functions require access to the same data.

The scope of a global variable is the entire program. This means that once a global variable is defined, it can be accessed and modified anywhere in the code. However, to modify the global variable within a function, it must be explicitly declared as global within that function, like so: ```python global_variable = 10 def foo(): global global_variable global_variable += 5 foo() print(global_variable) # Output: 15 ``` If you do not declare the global variable within the function, the modification will only affect a local variable with the same name, leaving the global variable unchanged. Remember that global variables should be used sparingly, as excessive use can lead to code that is difficult to understand and maintain. Whenever possible, prefer local variables and function arguments to share data between functions.