Common difference ($d=a_{k+1}-a_k$)

The common difference of an arithmetic progression is the fixed difference between consecutive sequence terms. It gives a finite difference that is independent of position. Checking one adjacent pair finds its value only when the arithmetic progression property is already known; otherwise, other pairs can disagree. For example, the differences in $7,10,14$ are three and four, so the three entries do not form an arithmetic progression.