A stratum is a layer of something, especially rock, soil, or other material built up over time. When geologists study a cliff face or dig into the ground, they can see different strata (the plural of stratum) stacked like layers in a cake. Each stratum tells a story about a different period in Earth's history. A dark stratum might show where ancient forests decayed, while a lighter one might reveal an era of volcanic activity.

Think of how sediment settles at the bottom of a river, year after year, forming distinct layers. Scientists can determine the age of fossils by identifying which stratum contains them, since lower strata are usually older than those above.

The word also describes layers in society or groups. Sociologists talk about different social strata, meaning groups of people with similar income, education, or status. A historian might study the various strata of a city's population to understand how different groups lived. In this sense, stratum helps us understand how things are organized in levels, whether we're talking about rock formations millions of years old or the structure of human communities.