Full Idea
Kripke's doctrine that natural kind words are rigid designators and our doctrine that they are indexical are two ways of making the same point.
Clarification
'Indexicals' point things out; 'rigid designators' baptise things for all possible worlds
Gist of Idea
Saying that natural kinds are 'rigid designators' is the same as saying they are 'indexical'
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Hilary Putnam - Meaning and Reference p.161
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.161
A Reaction
I think I prefer Putnam's terminology, because it is more modest in its claims Kripke gets into trouble when a natural kind in some other possible world is only subtly different from the original. How 'rigid'? Putnam sticks to how the word gets started.