橡胶 (xiàng jiāo) is the technical term for rubber as a raw material (e.g., natural or synthetic rubber used in tires, seals, and industrial products). 橡皮 (xiàng pí) is the everyday word that can also mean rubber material, but it most commonly refers to an eraser (for pencil). Use 橡胶 when talking about the material itself, especially in technical or industrial contexts.