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A global constant is a constant value that can be used throughout the entire program. It is declared outside any function or class, making it accessible from any part of the code. In programming languages like C++ and Java, global constants can be defined using the 'const' keyword, while in Python, it is just a regular variable written in uppercase letters by convention, to indicate that it should not be modified. For example, in C++: ```cpp const int GLOBAL_CONSTANT = 10; ``` In Python: ```python GLOBAL_CONSTANT = 10 ``` Keep in mind that in Python, constants are just a convention as the language itself does not enforce the immutability of the value.

Using global constants in a program is generally considered good practice when done responsibly. They can make the code more maintainable and readable by providing meaningful, descriptive names for values that are meaningful and used repeatedly throughout the program. They can also prevent potential errors caused by hardcoding the same value in multiple places, as you can update the value of the constant in one location. However, it's important to ensure that global constants are not abused. Constant values should not be overused, as this can lead to increased coupling between different parts of the code. Additionally, you should be careful to use meaningful names for global constants to make the code as readable as possible and avoid using global constants for values that are only used in one specific part of the program. In conclusion, it is permissible to use global constants in a program when done responsibly and with careful consideration of their impact on maintainability, readability, and potential coupling in the code.