British Dictionary definitions for arithmetic-series
arithmetic progression
noun
1.
a sequence of numbers or quantities, each term of which differs from the succeeding term by a constant amount, such as 3,6,9,12 Compare geometric progression
(ār'ĭth-mět'ĭk) A sequence of numbers such as 1, 3, 5, 7, 9 ..., in which each term after the first is formed by adding a constant (in this case, 2) to the preceding number. Compare geometric progression.