An arithmetic progression (AP) is a sequence of numbers in which each term, except the first, is obtained by adding a fixed number to the immediately preceding term. This fixed number is called the common difference, which is generally denoted by d.
For example: 1, 3, 5, 7 is an AP with common difference 2.
The series whose terms are in AP is called an arithmetic series.
For example: 1 + 3 + 5 + 7