A Stratum

1. One of several parallel horizontal layersof material arranged one on top of another.Synonym: tier
2. (geology) A layer of sedimentary rockhaving approximately the same composition throughout.Synonyms: bed, layerCoordinate term: seam
3. Any of the regions of the atmosphere, such as the stratosphere, that occur as layers.
4. (biology) A layer of tissue.
5. A class of society composed of peoplewith similar social, cultural, or economicstatus.
6. (ecology) A layer of vegetation, usually of similar height.
7. (computing) The level of accuracy of a computer's clock, relative to others on the network.