An arithmetic sequence is a list of numbers in which each term after the first is found by adding a constant value, known as the common difference, to the previous term. For example, in the sequence 2, 5, 8, 11, the common difference is 3, as each number increases by 3. The formula for the nth term of an arithmetic sequence can be expressed as a_n = a_1 + (n - 1) \cdot d , where a_n is the nth term, a_1 is the first term, d is the common difference, and n is the term number. This formula helps in finding any term in the sequence easily.