In a general sense, a theorem which proves that some proposition cannot logically be true. However, in economics the term is used specifically to refer to the theorem of K. J. Arrow. This shows that it is impossible to have a set of rules for a group (i.e. a constitution) which, while possessing certain minimum desirable properties, can con vert the preferences of the individual members of the group into a consistent set of preferences for the group as a whole.
Reference: The Penguin Dictionary of Economics, 3rd edt.