Full Idea
The definition of a class or collection which enumerates is called a definition by 'extension', and one which mentions a defining property is called a definition by 'intension'.
Gist of Idea
A definition by 'extension' enumerates items, and one by 'intension' gives a defining property
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], II)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.12
A Reaction
In ordinary usage we take intensional definitions for granted, so it is interesting to realise that you might define 'tiger' by just enumerating all the tigers. But all past tigers? All future tigers? All possible tigers which never exist?