A lens holder is a device used to securely hold lenses in place. It is typically made of plastic or wood and has adjustable screws that can be used to position the lens at the desired angle or distance. Lens holders are commonly used in optics experiments and are also useful for holding lenses during photography or videography. They come in various sizes and configurations to accommodate different types of lenses, and are an essential tool for anyone working with optics or lenses.