Hairlines, also known as hairline cracks, are tiny fractures that can appear on various surfaces, including skin, wood, ceramic, and metal. Besides hairlines, there are plenty of other synonyms to describe these fine lines. They can be called fissures, cracks, splits, or crevices, and they may appear as fine lines or deep crevices depending on their location and severity. Fine lines on the surface of paper or a drawing can be referred to as strokes or lines, while on a wall, they could be referred to as stress marks or cracks. Ultimately, the choice of synonym will depend on the context and the material in question.