An arithmetic sequence increases by a constant difference. Example: 2, 5, 8, 11 (common difference = 3).
nth term: a_n = a₁ + (n-1)d. Sum of n terms: S_n = n/2 × (2a₁ + (n-1)d)
A sequence where each term is formed by adding a constant (common difference d) to the previous term.