Tetragonal

Part of speech: adjective

Pronunciation: /tɛˈtɹæɡənəl/

Definitions

1. A geometric classification that involves a four-sided figure with equal lengths on opposing sides
2. Referring to a crystal lattice where the base is a rectangle and has perpendicular angles throughout its structure
3. A classification in geometry describing shapes with four equal sides arranged in pairs, commonly found in solid structures

Etymology: Ancient Greeks had a name for a four-cornered shape: "tetrágōnos", built from "téssares" ("four") and "gōnía" ("angle" or "corner"). The same stem gives us "tetragon", a plane figure with four angles, and from it English formed the adjective in "-al", which adds the sense "pertaining to" or "having the character of." So the adjective describes anything that has the qualities of a four-sided figure. In crystallography, the term describes a crystal system in which the unit cell is a square prism, or a rectangular prism with two equal axes, so that the angles stay right angles throughout the lattice. The word is a technical coinage built on classical roots, not an ancient term, and it belongs to the vocabulary of modern mineralogy.