[ 'plʌɪə(ʊ)si:n ]
■ adjective Geology relating to or denoting the last epoch of the Tertiary period (between the Miocene and Pleistocene epochs, 5.2 to 1.64 million years ago), a time when the first hominids appeared.
Origin
C19: from Gk pleiōn 'more' + kainos 'new'.