We present the new empirical parameter fc, the most probable usage
frequency of a word in a language, computed via the distribution of documents
over frequency x of the word. This parameter allows for filtering the core
lexicon of a language from the content words, which tend to be extremely
frequent in some texts written in specific genres or by certain authors.
Distributions of documents over frequencies for such words display long tails
as x>fc representing a bunch of documents in which such words are used in
abundance. Collections of such documents exhibit a percolation like phase
transition as the coarse grain of frequency Δf (flattening out the
strongly irregular frequency data series) approaches the critical value fc.Comment: RevTex, 4 pages, 2 figure