Full Idea
We can define a set by 'enumeration' (by listing the items, within curly brackets), or by 'abstraction' (by specifying the elements as instances of a property), pretending that they form a determinate totality. The latter is written {x | x is P}.
Gist of Idea
Sets can be defined by 'enumeration', or by 'abstraction' (based on a property)
Source
José L. Zalabardo (Introduction to the Theory of Logic [2000], §1.3)
Book Reference
Zalabardo,José L.: 'Introduction to the Theory of Logic' [Westview 2000], p.6