The S-N curve, also known as the Wöhler curve, is a graphical representation that illustrates the relationship between the number of cycles a material can endure before failure (N) and the applied stress level (S). It is commonly used in materials science and engineering to predict the fatigue life of materials under cyclic loading conditions. The curve typically has a characteristic S-shape, where the initial steep slope indicates a high failure rate at low cycles and high stress. As the number of cycles increases, the curve levels off, showing that materials can withstand lower stress levels for a significantly higher number of cycles before failure occurs.