---
tag: 4N7H08TX9R
kind: concept
name: Second
sources:
  - wikipedia: en:43580500@r1360947732
    locator: Glossary of set theory, section S
    note: adopted
    mention: wikipedia://en/43580500#7OST
    sections: [Summary]
inventory: [25JN49DFTE]
---

## Summary

A set of second category is a set that is not of first category: in other words a set that is not the union of a countable number of nowhere-dense sets.

An ordinal of the second class is a countable infinite ordinal

An ordinal of the second kind is a limit ordinal or 0

Second order logic allows quantification over subsets as well as over elements of a model