  A dictionary defines words in terms of other words. Definitions can tell you
the meanings of words you don't know, but only if you know the meanings of the
defining words. How many words do you need to know (and which ones) in order to
be able to learn all the rest from definitions? We reduced dictionaries to
their "grounding kernels" (GKs), about 10% of the dictionary, from which all
the other words could be defined. The GK words turned out to have
psycholinguistic correlates: they were learned at an earlier age and more
concrete than the rest of the dictionary. But one can compress still more: the
GK turns out to have internal structure, with a strongly connected "kernel
core" (KC) and a surrounding layer, from which a hierarchy of definitional
distances can be derived, all the way out to the periphery of the full
dictionary. These definitional distances, too, are correlated with
psycholinguistic variables (age of acquisition, concreteness, imageability,
oral and written frequency) and hence perhaps with the "mental lexicon" in each
of our heads.
