Define an arithmetic progression.

An arithmetic progression is a sequence of terms such that the difference between any two consecutive terms of the sequence is always same.

Suppose we have a sequence 

So, if these terms are in A.P., then,

$a_{2}-a_{1}=d$

$a_{3}-a_{2}=d$

$a_{4}-a_{3}=d$

And so on…

Here, d is the common difference of the A.P.

Example: 1, 3, 5, 7, 9 … is an A.P. with common difference (d) as 2.