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A global constant is a value that remains fixed throughout the entire program, i.e., its value doesn't change during the program's execution. It can be accessed from any part of the script or application, making it "global." In many programming languages, global constants are usually defined using the "const" keyword or in CAPITALIZED format to distinguish them from other variables.

It is not only permissible but also often recommended to use global constants in a program, especially when dealing with values that are not expected to change, such as mathematical constants (e.g., PI), configuration values (e.g., maximum array size), or constants set by the application's requirements. Using global constants helps in creating maintainable and less error-prone code because they prevent the accidental modification of values that should remain constant during the program's execution.

Some benefits of using global constants in a program include: 1. Readability & maintainability: Constants make the code more readable, as their purpose is clear, and their values are usually specified in one place, making it easier to maintain. 2. Preventing accidental modifications: As constants cannot be modified, using them prevents accidental changes to values that are supposed to remain fixed throughout the program. 3. Easier debugging: Constants make debugging easier, as their values aren't changed during the program's execution.

Although global constants can be beneficial to use, there are a few potential drawbacks: 1. Namespace pollution: Having too many global constants can cause clutter in the global namespace. This can lead to confusion and make the code harder to read. 2. Overuse: Using global constants for every constant value might not be practical in some cases. It's important to strike a balance between using global constants and other forms of variables/constants. In conclusion, using global constants in a program is not only permissible but also recommended in certain cases and can help create maintainable and efficient code. It's essential, however, to use them judiciously and avoid overusing them to the point of creating clutter in the global namespace.