1. A horizontal layer of material, especially one of several parallel layers arranged
one on top of another.
2. Geology: A bed or layer of sedimentary rock that is visually distinguishable from adjacent beds or layers.
3. Any of the regions of the atmosphere, such as the troposphere, that occur as layers.
4. Biology: A layer of tissue, the epithelial stratum.
5. A level of society composed of people with similar social, cultural, or economic status.
6. One of a number of layers, levels, or divisions in an organized system: a complex poem with many strata of meaning.
Latin strātum, a covering, from neuter past participle of sternere, to spread.
Source: http://www.thefreedictionary.com/stratum (derived, 15-7-2016)