Stratum (noun)

1. A layer of rock or soil, especially one of a series of distinct layers that make up the earth's crust.
2. A layer or level within a society or organization, especially one distinguished by a particular characteristic.

Origin:

Early 17th century: from latin, "layer, bed", past participle of sternere "spread out, strew".

Examples:

1. The rock stratum was rich in minerals.
2. The different strata of society have different levels of wealth and power.
3. The stratum of the atmosphere that contains ozone is called the ozone layer.
4. The lower strata of society are more affected by poverty.
5. The stratum of people who can afford to travel first class is very small.