Density gradient refers to the gradual change in density of a substance, often observed in mixtures or solutions. It occurs when different components have varying densities, causing them to separate into layers. For example, in a centrifuge, denser materials settle at the bottom while lighter ones rise to the top, creating a clear gradient. This concept is commonly used in scientific fields such as biology and chemistry to analyze samples. In a density gradient centrifugation, researchers can isolate specific cells or molecules based on their density, allowing for more precise studies and experiments.