These terms refer to thin treatments applied to the surface of lens elements or filters to improve performance.

• Coating: A single coating layer reduces reflections at the glass surface. Less reflected light means more light passes through, which can help reduce flare and improve contrast.
• Multi-coating: Light includes many wavelengths, and a single coating works best over a limited range. Multiple layers are used to reduce reflections across a broader range of wavelengths, so they are generally more effective.
• Nano-coating: This usually refers to an advanced surface structure or coating with extremely small physical features that further suppress reflections. Some nano-type coatings can also help repel water, grease, and dirt.

In general, coatings on lenses and filters are used to reduce unwanted reflections and improve transmission. Some coatings also add practical benefits such as easier cleaning or better resistance to moisture, smudges, and minor scratching.