Full Idea
Many categories are 'monothetic' (the defining set of features is always unique), and others are 'polythetic' (single features being neither essential to group membership nor sufficient to allocate an item to a group).
Gist of Idea
Monothetic categories have fixed defining features, and polythetic categories do not
Source
Roy Ellen (Anthropological Studies of Classification [1996], p.33)
Book Reference
Ellen,Roy: 'The Categorical Impulse' [Bergahn Books 2008], p.33
A Reaction
This seems a rather important distinction which hasn't made its way into philosophy, where there is a horrible tendency to oversimplify, with the dream of a neat and unified picture. But see Goodman's 'Imperfect Community' problem (Idea 7957).
Related Idea
Idea 7957 Without respects of resemblance, we would collect blue book, blue pen, red pen, red clock together [Goodman, by Macdonald,C]