Sequence
For example, the rule could be that the thing in the n-th place is the number 2×n (2 times n). This tells us what the whole sequence is, even though it never ends. The first number is 2×1, which is 2. The second number is 2×2, or 4. If we want to know what the 100-th number is, we can simply calculate 2×100 and get 200. No matter which thing in the sequence we want, the rule can tell us what it is.