strata, stratum

A stratum (Latin for “something spread”) is a layer or a social class.

• Which stratum of society benefits most from a flat tax? The very rich.

Strata is the plural of stratum; stratas is a non-standard plural form.

• Which strata (not stratas) of society benefit from a flat tax? The rich and the very rich, according to its opponents.
• It was easy to distinguish the three strata (not stratas) of metamorphic rock.

As a compound modifier, strata is connected with a hyphen to the word before or after it to avoid ambiguity.

• The research on atomic energy was based on a double-strata fuel cycle.
• The geologists extracted samples from the strata-bound ore field.