> So far, we've informally been dealing with lists (of numbers, but equally strings) and single items (letters or numbers).
In reality, these are two instances of a larger family of arrays. In APL, even single atomic pieces of data (like a letter or a number) is an array, but since it is lone, you don't need any indices to pick out the data. We call it zero-dimensional, or "rank 0". List need a single index to pick out an item, so we call them 1-dimensional, or "rank 1".