The index of a number says how many times to use the number in a multiplication.
It is written as a small number to the right and above the base number.
In this example the index is 2: 82 = 8 × 8 = 64
(It says to use 8 2 times in a multiplication)
Another example: 53 = 5 × 5 × 5 = 125
The plural of index is indices.
(Other names for index are exponent or power.)