The term quaternary is derived from the Latin word for four, and it refers to a group or set of four elements or parts. It is commonly used in geology, where it is used to describe a period of time that is divided into four distinct parts. In biology, quaternary structure is used to describe the arrangement of four subunits in a protein molecule. In mathematics, quaternary numbers are used to represent four-digit numbers. Quaternary can also be used to refer to a fourth-order system or process.