British Dictionary definitions for arithmetic-series

arithmetic progression

noun

a sequence of numbers or quantities, each term of which differs from the succeeding term by a constant amount, such as 3,6,9,12Compare geometric progression

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.