The "assert.h" header file in C programming provides a way to diagnose logical errors in a C program during runtime. It defines the "assert" macro, which takes an expression as its argument and verifies if the expression is true. If the expression is false, the "assert" macro causes the program to terminate and prints an error message to the standard error stream.

The syntax of the "assert" macro is as follows:

void assert(int expression);

The "assert" macro takes an integer expression as its argument. If the expression is true (non-zero), the macro does nothing. However, if the expression is false (zero), the macro prints an error message to the standard error stream and calls the "abort" function to terminate the program.

Here is an example of using the "assert" macro in a C program:

#include <stdio.h>
#include <assert.h>

int main() {
    int x = 10;
    int y = 20;

    // Verify that x is less than y
    assert(x < y);

    // This line will not be executed if the assertion fails
    printf("x is less than y\n");

    return 0;
}

In this example, the "assert" macro is used to verify that the variable "x" is less than the variable "y". If this condition is false, the program will terminate and print an error message. If the condition is true, the program will continue to execute normally.