Fatigue refers to damage accumulated through repeated cyclic stresses over time. It is affected by factors like stress type/amplitude, surface finish, material properties, and environment. Fatigue life depends on the mean and amplitude of stresses based on relationships like the Goodman diagram. The Miner's rule states that the fraction of fatigue life consumed by stresses must be less than 1 to avoid failure. Fatigue tests involve repeated cyclic loading in tension, compression, bending, or other patterns to determine fatigue properties.