The Cretaceous (pronounce /kriːˈteɪʃəs/, usually abbreviated 'K' for its German translation "Kreide") is a geological period, reaching from the end of the Jurassic Period, (145.5 ± 4 million years ago (Ma) to the beginning of the Paleocene Period, 65.5 ± 0.3 Ma. It is the youngest geological period of the Mesozoic, and at 80 million years long, the longest period of the Phanerozoic. The end of the Cretaceous defines the boundary between the Mesozoic and Cenozoic eras.