Let's imagine, instead of numbers, we have a sequence of animals. The sequence begins with three animals: green, tap dancing elephant, purple, singing cockatoo, and orange, quarterback tiger. We could refer to the first term as green, tap dancing elephant, the second as the purple, singing cockatoo, and so on. But we will just become confused in our circus of exotic animal talent.
Or, we could just give each of sequence a short label.
We call the first term of a sequence a1 (the elephant in our circus), the second term a2 (the cockatoo), and so on:
a1, a2, a3, ..., an, ....
The nth term, an, is called the general term of a sequence.
Sometimes it's useful to think of a sequence as starting with a "zeroth" term instead of a "first" term. Then we would think of the sequence as
a0, a1, a2, a3, ..., an, ....
It sounds like we are just being difficult, defining a term by a number that means none. We aren't. We'll tell you why later. For now, assume sequences start with n = 1 unless told otherwise.
With sequences, we like to know how to get there from here. To describe or define a sequence we need to know how to find the general term. It's like knowing how to get from your couch, where you were reading up on Greek mythology for giggles, to the kitchen to fetch a slice of watermelon.
This description may or may not involve a mathematical formula. Some sequences can't be described by a nice tidy formula.
Define a sequence by
an = high temperature in Mountain View, CA on the nth day after January 1, 2010.
This is a perfectly reasonable definition. However, as far as anyone knows, there is no mathematical formula that can produce the nth term. Despite our extensive knowledge of rain dancing, we can't predict the weather that accurately.