"Insoluble" refers to a substance's inability to dissolve in a solvent, typically water. When a material is insoluble, it does not mix with the solvent at a molecular level, resulting in the formation of a separate phase, such as solid particles suspended in a liquid. Common examples of insoluble substances include sand and certain minerals. In various contexts, such as chemistry and environmental science, understanding solubility is crucial for applications like filtration, separation, and chemical reactions.