"Crystalline" refers to a solid material whose atoms are arranged in a highly ordered, repeating pattern. This structure gives crystalline substances distinct shapes and properties, such as sharp edges and flat surfaces. Common examples of crystalline materials include salt, sugar, and diamonds. The arrangement of atoms in a crystalline structure can affect its physical properties, such as melting point and hardness. Crystalline materials can be found in nature, like quartz and ice, or can be synthesized for various applications, including electronics and pharmaceuticals.