The "stddef.h" header file in C programming defines several macros and types that provide basic definitions used in C programs, such as null pointers and size_t.

Some of the commonly used definitions in "stddef.h" include:

1. "NULL": A macro that expands to a null pointer constant.

2. "size_t": An unsigned integer type that is used to represent the size of an object.

3. "ptrdiff_t": A signed integer type that is used to represent the difference between two pointers.

4. "offsetof": A macro that evaluates the offset of a member in a struct or union.

Here is an example of using the "stddef.h" header file in a C program:

#include <stdio.h>
#include <stddef.h>

struct example {
    int x;
    char y;
    double z;
};

int main() {
    size_t struct_size = sizeof(struct example);
    printf("Size of struct example: %zu\n", struct_size);

    size_t offset_y = offsetof(struct example, y);
    printf("Offset of member y: %zu\n", offset_y);

    return 0;
}

In this example, the "stddef.h" header file is used to get the size of a struct and the offset of a member within that struct. The "sizeof" operator is used to get the size of the struct, and the "offsetof" macro is used to get the offset of the "y" member within the struct. The output of the program will be:

Size of struct example: 24
Offset of member y: 4

Note that the behavior of the definitions in "stddef.h" is platform-dependent, and they may differ between different implementations of C.