Full Idea
Natural kinds can be associated with 'strong' stereotypes (giving a strong picture of a typical member, like a tiger), or with 'weak' stereotypes (with no idea of a sufficient condition, such as molybdenum or elm).
Gist of Idea
Natural kind stereotypes are 'strong' (obvious, like tiger) or 'weak' (obscure, like molybdenum)
Source
Hilary Putnam (Explanation and Reference [1973], II C)
Book Reference
Putnam,Hilary: 'Mind Language and Reality: Papers vol 2' [CUP 1975], p.205