Hereditarily finite set
Set
definiendum | Vω in it |
postulate | ∅∈Vω |
for all | x∈Vω |
postulate | P(x)∈Vω |
postulate | x=∅ ∨ ∃(y∈Vω). x=P(y) |
Discussion
Idea
This is the set of all finite sets constructable when starting with ∅. It's the smallest infinite Grothendieck universe, as well as a model of ZFC.
Reference
Wikipedia: Hereditarily finite set