- Ordered list of numbers.
- eg: 1, 3, 5, 7, 9 ......
- $1, \frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \frac{1}{5} $ ......
- $a_1, a_2, a_3, \ldots a_n$, .......
- $a_i$'s are numbers.
- $1 \rightarrow a_1$
- $2 \rightarrow a_2$
- $3 \rightarrow a_3\ldots\ldots$