The "Density of States" (DOS) is a concept in physics that describes the number of available quantum states for particles, such as electrons, at each energy level within a material. It helps to understand how many states are available for occupation by particles, influencing the material's electrical and thermal properties. In solid-state physics, the DOS is crucial for analyzing the behavior of materials, including metals, semiconductors, and insulators. It is often represented graphically, showing how the number of states varies with energy, which can affect phenomena like conductivity and heat capacity.