In an arithmetic progression (AP), "n" refers to the number of terms in the sequence. It is used in the formula for finding the sum of the first "n" terms of an AP, which is given by: S = n/2[2a + (n-1)d], where "a" is the first term and "d" is the common difference between consecutive terms. "n" can also be used to find the nth term of an AP using the formula an = a + (n-1)d, where "a" is the first term and "d" is the common difference.