Quaternary

Quaternary is an adjective meaning “fourth” or “containing four parts,” from Latin quaternarius. It is used across disciplines to denote the fourth in a series, a four-part structure, or a four-dimensional concept.

In geology and paleoanthropology, the Quaternary is the current and most recent geological time period, extending from about 2.58 million years ago to the present. It follows the Neogene and is characterized by climate fluctuations between glacials and interglacials. The period is subdivided into the Pleistocene, marked by repeated ice ages, and the Holocene, the current epoch that began roughly 11,700 years ago with the end of the last major glaciation. The Quaternary also encompasses the rise and spread of humans and significant climatic and environmental changes.

In chemistry, the term appears in several contexts. Quaternary ammonium compounds feature a nitrogen atom bonded to four carbon-containing groups and carry a positive charge, as in tetramethylammonium salts or benzalkonium chloride. These compounds are widely used as surfactants and disinfectants. The term also describes a quaternary carbon, a carbon atom bonded to four other carbon atoms, a common feature in branched and heavily substituted organic molecules. The process of forming quaternary ammonium species is called quaternization.

In mathematics and related fields, quaternions form a four-dimensional number system extending complex numbers, consisting of 1, i, j, and k with noncommuting multiplication. Quaternions are important in three-dimensional rotations and computer graphics.

In biology, quaternary structure refers to the assembly of multiple polypeptide subunits into a functional protein complex, as seen in hemoglobin and many enzymes.

The term can also refer to the base-4 (quaternary) numeral system, which uses four digits and is used mainly in theoretical or specialized computational contexts.