hereditarily countable set
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In set theory, a set is called hereditarily countable if it is a countable set of hereditarily countable sets.
Results
The inductive definition above is well-founded and can be expressed in the language of first-order set theory.
=Equivalent properties=
A set is hereditarily countable if and only if it is countable, and every element of its transitive closure is countable.[https://www.jstor.org/pss/2273380 "On Hereditarily Countable Sets"] by Thomas Jech.