Statistical studies of languages have focused on the rank-frequency
distribution of words. Instead, we introduce here a measure of how word ranks
change in time and call this distribution \emph{rank diversity}. We calculate
this diversity for books published in six European languages since 1800, and
find that it follows a universal lognormal distribution. Based on the mean and
standard deviation associated with the lognormal distribution, we define three
different word regimes of languages: "heads" consist of words which almost do
not change their rank in time, "bodies" are words of general use, while "tails"
are comprised by context-specific words and vary their rank considerably in
time. The heads and bodies reflect the size of language cores identified by
linguists for basic communication. We propose a Gaussian random walk model
which reproduces the rank variation of words in time and thus the diversity.
Rank diversity of words can be understood as the result of random variations in
rank, where the size of the variation depends on the rank itself. We find that
the core size is similar for all languages studied
