a layer of material, naturally or artificially formed, often one of a number of parallel layers one upon another

a stratum of ancient foundations.

one of a number of portions or divisions likened to layers or levels

an allegory with many strata of meaning.

Geology . a single bed of sedimentary rock, generally consisting of one kind of matter representing continuous deposition.

Biology . a layer of tissue; lamella.

Ecology . (in a plant community) a layer of vegetation, usually of the same or similar height.

a layer of the ocean or the atmosphere distinguished by natural or arbitrary limits.

Sociology . a level or grade of a people or population with reference to social position, education, etc.

the lowest stratum of society.

Linguistics . (in stratificational grammar) a major subdivision of linguistic structure. Compare level ( def. 17 ) .

noun

bed , gradation , grade , lamina , level , lode , seam , sheet , stratification , thickness , vein , class , division , group , layer , tier