1. Any of several parallel levels of something: Along the east coast of Greenland, the strata of the cliffs by the ocean were very interesting for the group of tourists to see.
2. A layer of the atmosphere or the sea: One stratum or level of the air or sky is the troposphere, which occurs in tiers or heights.
3. In biology, a fold of living cells; or a sheetlike mass of substance of nearly uniform thickness; especially, when it is one of several associated levels, as in tissue: In Robert’s biology class at school, he learned that one of several layers of skin tissue in a body is known as the epithelial strata.
4. A horizontal layer of vegetation within a stratified plant area: In the Alps above the tree line, there is only a selected stratum of plant life that can survive such extreme climate changes.
5. A social rank or level of society consisting of people of similar cultural, economic, or educational status: There are many different political viewpoints within one stratum, or social class, of individuals in the town where Joan’s sister lives.
6. A layer or level within an ordered system: In the poem that Jane was reading for school, she had to find out the different strata of meanings and thoughts it conveyed to the reader. © ALL rights are reserved.

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Horizontal layers of sedimentary or igneous rock that produces, contains, or is impregnated with salt: Some wild plant species, called the saltworts or halophytes, are quite adapted to living in an environment of inorganic matter containing a saliferous stratum.

Layer upon layer.