Full Idea
It is a matter of chance which characteristics of a man's appearance will come into a friend's mind when he thinks of Bismarck; thus the description in the friend's mind is accidental; he knows the various descriptions all apply to the same entity.
Gist of Idea
It is pure chance which descriptions in a person's mind make a name apply to an individual
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 5)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.30
A Reaction
This seems to be an internalist account of reference, later called the 'bundle' theory of reference and associated with John Searle. It was attacked by Kripke. Personally I side, unfashionably, with Russell.