Stratum

One of several parallel horizontal layers of material arranged one on top of another
(geology) A layer of sedimentary rock having approximately the same composition throughout
Any of the regions of the atmosphere, such as the stratosphere, that occur as layers
(biology) A layer of tissue
A class of society composed of people with similar social, cultural, or economic status
(ecology) A layer of vegetation, usually of similar height.

복수: Strata