A global variable in C is a variable that is defined outside of any function and can be accessed by any function in the program. Global variables have a global scope, which means they can be accessed from anywhere within a program. They are usually declared before the main function.

Here are some key points about global variables in C:

• Global variables can be accessed and modified by any function in the program.
• Global variables are allocated memory in the data segment of the program, which means they are allocated at compile-time and remain in memory for the entire duration of the program.
• Global variables can be useful when we need to share data between functions or when we need to keep track of a variables value over time.
• Global variables can simplify code.
• Global variables can cause unaccounted problems in programs because their values can be changed by any function.
• It is generally advised to avoid using global variables.

To declare a global variable in C, we simply declare it outside of any function using the following syntax:

int x; // global variable declaration

Overall, global variables can be a powerful tool in programming, but they should be used with care.