Stratum

A bed of earth or rock of one kind, formed by natural causes, and consisting usually of a series of layers, which form a rock as it lies between beds of other kinds. Also used figuratively.

A bed or layer artificially made; a course.

an abstract place usually conceived as having depth; "a good actor communicates on several levels"; "a simile has at least two layers of meaning"; "the mind functions on many strata simultaneously"

a subpopulation divided into a stratified sampling

people having the same social, economic, or educational status; "the working class"; "an emerging professional class"

one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)