Definition of: stratum

(strā′təm, strat′əm) noun plural ·ta (-tə) or ·tums
1. A natural or artificial layer, bed, or thickness.
2. Geol. A more or less homogeneous layer of rock, often in two or more beds, and serving to identify a geological group, system, or series.
3. Biol. A sheet or layer of tissue.
4. Something corresponding to a stratum of the earth: a low stratum of society. [<L, orig. neut. of stratus, pp. of sternere spread]