A stratum is a layer of something, whether abstract, like a social stratum composed of only billionaires, or physical, like the different strata (that’s the plural version) of clouds that an airplane passes through.

Stratum comes from the Latin meaning "something that has been laid down," like asphalt or a bedsheet, but we generally use it to describe layers of something. If you go into geology, you will probably investigate different strata of the soil. If you start out in life poor and try to become rich, you are trying to get into a different social stratum.

Definitions of stratum
  1. noun
    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)
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    cuticle, epidermis
    the outer layer of the skin covering the exterior body surface of vertebrates
    corneum, horny layer, stratum corneum
    the outermost layer of the epidermis consisting of dead cells that slough off
    stratum lucidum
    the layer of epidermis immediately under the stratum corneum in the skin of the palms and soles
    stratum granulosum
    the layer of epidermis just under the stratum corneum or (on the palms and soles) just under the stratum lucidum; contains cells (with visible granules) that die and move to the surface
    malpighian layer, rete Malpighii, stratum basale, stratum germinativum
    the innermost layer of the epidermis
    corium, derma, dermis
    the deep vascular inner layer of the skin
    the inner layer of the periosteum
    paries, wall
    (anatomy) a layer (a lining or membrane) that encloses a structure
    substrate, substratum
    any stratum or layer lying underneath another
    superstrate, superstratum
    any stratum or layer superimposed on another
    a specific layer or stratum of soil or subsoil in a vertical cross section of land
    bed, seam
    a stratum of ore or coal thick enough to be mined with profit
    (geology) a stratum of rock (especially sedimentary rock)
    mantle, pallium
    (zoology) a protective layer of epidermis in mollusks or brachiopods that secretes a substance forming the shell
    abdominal wall
    a wall of the abdomen
    soil horizon
    a layer in a soil profile
    geological horizon
    a layer of rock with a particular composition (especially of fossils); for dating the stratum
    coal seam
    a seam of coal
    ore bed
    a stratum of ore
    type of:
    a relatively thin sheetlike expanse or region lying over or under another
  2. noun
    people having the same social, economic, or educational status
    synonyms: class, social class, socio-economic class
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    domain, world
    people in general; especially a distinctive group of people with some shared interest
    age class
    people in the same age range
    the class of people engaged in growing food
    brotherhood, fraternity, sodality
    people engaged in a particular occupation
    estate, estate of the realm, the three estates
    a major social class or order of persons regarded collectively as part of the body politic of the country (especially in the United Kingdom) and formerly possessing distinct political rights
    labor, labour, proletariat, working class
    a social class comprising those who do manual labor or work for wages
    lower class, underclass
    the social class lowest in the social hierarchy
    bourgeoisie, middle class
    the social class between the lower and upper classes
    class consisting of all those who are considered boobs
    commonality, commonalty, commons
    a class composed of persons lacking clerical or noble rank
    the class of peasants
    a class of woman not considered respectable because of indiscreet or promiscuous behavior
    the criminal class
    class of small freeholders who cultivated their own land
    a social class separated from others by distinctions of hereditary rank or profession or wealth
    (Hinduism) a hereditary social class among Hindus; stratified according to ritual purity
    upper class, upper crust
    the class occupying the highest position in the social hierarchy
    a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations
    firing line
    the most advanced and responsible group in an activity
    immigrant class
    recent immigrants who are lumped together as a class by their low socioeconomic status in spite of different cultural backgrounds
    politically moderate persons; centrists
    old school
    a class of people favoring traditional ideas
    the customers for a particular product or service
    craft, trade
    people who perform a particular kind of skilled work
    fair sex, woman, womanhood
    women as a class
    Lords Spiritual, first estate
    the clergy in France and the heads of the church in Britain
    Lords Temporal, second estate
    the nobility in France and the peerage in Britain
    Commons, third estate
    the common people
    fourth estate
    the press, including journalists, newspaper writers, photographers
    labor force, labor pool
    the source of trained people from which workers can be hired
    (Marxism) the unorganized lower levels of the proletariat who are not interested in revolutionary advancement
    organized labor
    employees who are represented by a labor union
    petit bourgeois, petite bourgeoisie, petty bourgeoisie
    lower middle class (shopkeepers and clerical staff etc.)
    academe, academia
    the academic world
    Grub Street
    the world of literary hacks
    (Hinduism) a Hindu caste or distinctive social group of which there are thousands throughout India; a special characteristic is often the exclusive occupation of its male members (such as barber or potter)
    elite, elite group
    a group or class of persons enjoying superior intellectual or social or economic status
    aristocracy, gentry
    the most powerful members of a society
    people in power, ruling class
    the class of people exerting power or authority
    black market
    people who engage in illicit trade
    type of:
    (plural) any group of human beings (men or women or children) collectively
  3. noun
    an abstract place usually conceived as having depth
    “the mind functions on many strata simultaneously”
    synonyms: layer, level
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    type of:
    an abstract mental location
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