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A global constant is a variable or object that is declared outside of any function or class and is accessible to any part of the code. It is typically declared with the keyword `const` or `constexpr`, which means its value cannot be changed once it has been initialized.

Yes, it is permissible to use global constants in a program. In fact, using global constants can be beneficial in various ways: 1. It helps to maintain the same value consistently across the whole program which may prevent unexpected bugs and errors. 2. It can make the code more readable, as the constant's value and purpose are clear, even when its value isn't immediately recognizable. 3. It can help to improve code maintainability, as changes can be made to the constant value in one place which will then propagate throughout the code. However, it is important to limit the use of global variables in a program (variables that are not constants) as they can lead to unexpected changes in value, making the code harder to debug and maintain. Additionally, it is best to use local constants within functions or namespaces instead of global constants when their usage is limited to a specific scope.