stratum
     n 1: one of several parallel layers of material arranged one on
          top of another (such as a layer of tissue or cells in an
          organism)
     2: 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" [syn: {level}, {layer}]
     [also: {strata} (pl)]
Source: WordNet® 2.0