Full Idea
The Peano-Russell notion of class is the 'logical' notion, where each collection is associated with some kind of definition or rule that characterises the members of the collection.
Gist of Idea
The 'logical' notion of class has some kind of definition or rule to characterise the class
Source
Shaughan Lavine (Understanding the Infinite [1994], IV.1)
Book Reference
Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.63