core + attic (Attic Area): these terms have been either deprecated from the core or extentions, or removed from pending as not accepted into the full vocabulary.

citation

Canonical URL: http://schema.org/citation

Thing > Property > citation

A citation or reference to another creative work, such as another publication, web page, scholarly article, etc.

Usage: Between 100 and 1000 domains
Values expected to be one of these types
CreativeWork
Text
Used on these types
CreativeWork

Schema Version 3.4