When you declare an array for example:
const int size=4;
what is the correct word to use to describe the  in the array??
well in the line:
the  just indicates it is an array and the value inside the square brackets is the size of the array.
When used like this:
the  represents the index into the array.
Indices (plural of index).
Or if you mean the symbols themselves
So in array
5 is the number of indices, array is a reference to it's fourth index, and the numbers are surrounded by brackets.