i

One thing you will very often see if you read other peoples' code is that the variable being tracked in a for loop is often called i.

void main() {
String word = "bird";

for (int i = 0; i < array.length; i++) {
    char letter = word.charAt(i);
    System.out.println(letter);
}

// b
// i
// r
// d
}

While usually naming variables with single letters isn't a great idea, most developers carve out an exception for cases like this. Writing index gets tedious.

Its also helpful to go i -> j -> k when you end up nesting for loops.¹

void main() {
char[] letters = { 'A', 'B', 'C' };
int[] numbers = { 1, 2 };

for (int i = 0; i < letters.length; i++) {
    for (int j = 0; j < numbers.length; j++) {
        System.out.print(letters[i]);
        System.out.println(numbers[j]);
    }
}

// A1
// A2
// B1
// B2
// C1
// C2
}

Just do not start naming all your variables single letters.