The ratio test is a method used in calculus to determine the convergence or divergence of infinite series. It involves examining the limit of the absolute value of the ratio of consecutive terms in the series. If this limit is less than 1, the series converges; if it is greater than 1, the series diverges. If the limit equals 1, the test is inconclusive. To apply the ratio test, you take a series represented as \sum a_n and compute L = \lim_n \to \infty \left| \fraca_{n+1}a_n \right| . This helps mathematicians analyze the behavior of series, especially those involving factorials or exponential functions.