stratum

Short definition from Wiktionary

Noun

1. 1
   One of several parallel horizontal layers of material arranged one on top of another.
2. 2
   A layer of sedimentary rock having approximately the same composition throughout.
3. 3
   Any of the regions of the atmosphere, such as the stratosphere, that occur as layers.
4. 4
   A layer of tissue.
5. 5
   A class of society composed of people with similar social, cultural, or economic status.
6. 6
   A layer of vegetation, usually of similar height.
7. 7
   The level of accuracy of a computer's clock, relative to others on the network.
8. 8
   A historical layer of a language.