"Glaciation cycles" refer to the natural and repetitive process of ice sheets growing and receding on the Earth's surface over long periods of time. Full definition
A geologist's revisionist theory pushing the formation of the Isthmus of Panama back 10 million years casts doubt on mainstream ideas of what caused the last ice age as well as the global glaciation cycle that generates the world's current climate (scientificamerican.com)
BTW: going back 15 million years or more (say, to dinos 65 Myr) is not relevant to GHG warming today — the current glaciation cycle is only a COUPLE of a million years old! (dotearth.blogs.nytimes.com)
Tropical sea surface temperatures were controlled more by atmospheric connections to glaciation cycles than by ocean circulation. (science.sciencemag.org)