rhomboid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A parallelogram with unequal adjacent sides.
• adjective Shaped like a rhombus or rhomboid.

from The Century Dictionary.

• Having a form like or approaching that of a rhomb; having the shape of a rhomboid (see II., 1); rhomboidal.
• noun In geometry, a quadrilateral figure whose opposite sides and angles are equal, but which is neither equilateral nor equiangular; a non-equilateral oblique parallelogram.
• noun In crystallography, a solid having a rhomboidal form with three axes of unequal lengths, two of which are at right angles to each other, while the third is so inclined as to be perpendicular to one of the two axes, and oblique to the other.
• noun In anatomy, a rhomboideus.
• noun In anthropology, a cranium the norma verticalis of which has a rhomboid form.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) An oblique-angled parallelogram like a rhomb, but having only the opposite sides equal, the length and with being different.
• adjective Same as rhomboidal.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A parallelogram which is neither a rhombus nor a rectangle
• noun Any of several muscles that control the shoulders
• noun A solid shape which has rhombic faces
• adjective resembling, or shaped like a rhombus or rhomboid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of several muscles of the upper back that help move the shoulder blade
• noun a parallelogram with adjacent sides of unequal lengths; an oblique-angled parallelogram with only the opposite sides equal
• adjective shaped like a rhombus or rhomboid