A stratum, or layer, refers to a distinct level or layer within a system, often used in various fields such as geology, biology, and sociology. In geology, strata are layers of rock or sediment that have formed over time, each representing a specific period in Earth's history. These layers can provide valuable information about the environment and conditions that existed when they were formed. In biology, the term stratum can describe different layers of cells or tissues within an organism, such as the epidermis in plants or the stratum corneum in human skin. In sociology, strata can refer to social classes or groups within a society, highlighting the differences in wealth, education, or power among individuals.