The Pleistocene was an era (TV: Time-Flight) of Earth history, alternately referred to as a period. (TV: The Seeds of Doom)
When sent back 140,000,000 years to the Jurassic, the Fifth Doctor believed the chill in the air indicated the Pleistocene was not far off. (TV: Time-Flight)
Bernice Summerfield knew of a theory that humans were reduced to less than ten thousand individuals during the Pleistocene; she believed a single drought could have wiped them out. (PROSE: Genius Loci)
A Krynoid pod found in Antarctica dated to 18,000 BC was from the late Pleistocene. (TV: The Seeds of Doom)
Behind the scenes
Contrary to the Fifth Doctor in Time-Flight, the Pleistocene epoch was over 143,000,000 years after the Jurassic period. Furthermore, 140,000,000 years ago was in the early Cretaceous, not the Jurassic.
While not mentioned in the text, COMIC: Medicine Man is dated to the Pleistocene on the Titan Comics website.[1]