Quaternary ( /kwəˈtɜːrnəri/) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.588 ± 0.005 million years ago to the present. The Quaternary Period is divided into two epochs: the (2.588 million years ago to 11.7 thousand years ago) and the Holocene (11.7 thousand years ago to today). The informal term "Late Quaternary" refers to the past 0.5–1.0 million years.
The Quaternary Period is typically defined by the cyclic growth and decay of continental ice sheets driven by Milankovitch cycles and the associated climate and environmental changes that occurred.
The term Quaternary ("fourth") was proposed by Giovanni Arduino in 1759 for alluvial deposits in the Po River valley in northern Italy. It was introduced by Jules Desnoyers in 1829 for sediments of France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks.
The Quaternary Period follows the Neogene Period and extends to the present. The Quaternary covers the time span of glaciations classified as the , and includes the present interglacial time-period, the Holocene.