What does trigonal mean?
The adjective trigonal is chiefly used in geometry and crystallography to describe forms related to three angles or, more precisely in crystal science, an arrangement with threefold rotational symmetry. In a broad geometric sense, it refers to something shaped like or constituting a triangle. Its specialized technical sense is important in mineralogy and solid-state chemistry: crystals in the trigonal crystal system have a lattice distinguished by a threefold axis, even though an individual specimen may look far from triangular. In molecular geometry, “trigonal planar” describes a central atom bonded to three surrounding atoms in one plane. The word is formal and scientific rather than everyday, so “triangular” is normally preferable when merely describing a three-sided shape. Trigonal remains useful because it identifies a specific symmetry or structural category, not simply a visual resemblance.
adjectiveRelating to or having the form of a triangle. In crystallography, it denotes a structure characterized by threefold rotational symmetry.
- Relating to a triangle or having a three-angled form.
- Having threefold rotational symmetry; belonging to the trigonal crystal system.
"The molecule adopts a trigonal planar arrangement, with three bonds extending evenly around the central atom."
"The mineral develops distinct crystals within the trigonal crystal system."
"A trigonal planar molecule places three surrounding atoms at equal angular intervals in one plane."
The same word that describes a triangle also names one of the seven crystal systems.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does trigonal come from?
Trigonal comes through Latin from Greek trigōnos, meaning “three-angled.” The Greek combines tri- (“three”) with gōnía (“angle”), a root also underlying triangle and the field of trigonometry. The English adjective developed in geometry and later became an established crystallographic term.
Related word forms
How trigonal is actually used
The word is formal and technical, especially in geometry, crystallography, mineralogy, and chemistry. When simply describing a shape with three sides, “triangular” is generally more natural.
Easily confused with trigonal
“Triangular” broadly describes a three-sided figure, whereas “trigonal” can specifically identify a threefold rotational symmetry in crystallography.