stratum
/ˈstrɑ:təm; ?@ ˈstreɪtəm; `stretəm/ n
(pl strata /-tE; -tE/)
1 any of a series of horizontal layers, esp of rock in the earth's crust 层; （尤指）岩层.
2 level or class in society （社会的）阶层
a gathering of people from a variety of social strata 不同社会阶层的人的集会.