disphenoid
Definition from Wiktionary, the free dictionary
English[edit]
Etymology[edit]
From di (“twice, double”) + sphenoid (“wedgeshaped crystal or bone of the skull”).
Adjective[edit]
disphenoid (not comparable)
 (mineralogy) Of or pertaining to a wedgeshaped crystal form of the tetragonal or orthorhombic system.
 (mineralogy) Of or pertaining to a crystal form bounded by eight scalene triangles arranged in pairs, constituting a tetragonal scalenohedron.
Noun[edit]
disphenoid (plural disphenoids)
 (geometry) A nonregular tetrahedron whose four faces are congruent acuteangled triangles.

1949, Edward Salisbury Dana, Minerals and How to Study Them^{[1]}, page 32:
 The mineral chalcopyrite commonly crystallizes in disphenoids that are very difficult to distinguish from tetrahedrons.
 1973, H. S. M. Coxeter, 3rd Edition, unnumbered page,
 To make a model of a disphenoid, cut out an acute angled triangle and fold it along the joins of the midpoints of the sides. The disphenoid is said to be rhombic or tetragonal according as the triangle is isosceles or scalene.
 1977, Elizabeth A. Wood, Crystals and Light: An Introduction to Optical Crystallography, 2nd Revised Edition, page 8,
 If you rotate the [tetragonal] disphenoid 90° around its 2fold axis and then perform the operation of inversion through the centerpoint of the object, it will occupy its original position again.
 1993, Horst Martini, A Hierarchical Classification of Euclidean Polytopes with Regularity Properties, Tibor Bisztriczky, Peter McMullen, Rolf Schneider, Asia Ivic Weiss (editors), Polytopes: Abstract, Convex and Computational, Proceedings of the NATO Advanced Study Institute, page 83,
 In addition it should be remarked that there are two types of disphenoids, with different symmetries: the tetragonal disphenoid (having isosceles facets) and the rhombic one.

Synonyms[edit]
 (nonregular tetrahedron with congruent faces): bisphenoid, equifacial tetrahedron, isosceles tetrahedron