In C++, an if statement can be nested inside another if statement, allowing for more complex conditional logic to be expressed. A nested if statement is simply an if statement that is contained within another if or else block.

Here's an example of a nested if statement:

int x = 10;
if (x > 5) {
    if (x < 15) {
        cout << "x is between 5 and 15" << endl;
    } else {
        cout << "x is greater than or equal to 15" << endl;
    }
} else {
    cout << "x is less than or equal to 5" << endl;
}

In this example, the outer if statement checks if x > 5, which is true because x is equal to 10. The inner if statement then checks if x < 15, which is also true, so the message "x is between 5 and 15" is printed to the console.

Nested if statements can be nested to any depth needed, but it's important to keep the code organized and easy to read. In general, it's a good practice to avoid deeply nested if statements, as they can become difficult to understand and maintain. If you find that your code is becoming too complex, you may want to consider refactoring it into smaller, more manageable pieces.