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A rhombus is a parallelogram whose sides are all the same length.


Its angles may or may not all be equal.

Also known as

A rhombus is also known as a rhomb.

Particularly when it is oriented so that the longer diagonal is a vertical line, a rhombus is often referred to as a diamond.

Also see

Euclid's Definitions

In the words of Euclid:

Of quadrilateral figures, a square is that which is both equilateral and right-angled; an oblong that which is right-angled but not equilateral; a rhombus that which is equilateral but not right-angled; and a rhomboid that which has its opposite sides equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called trapezia.

(The Elements: Book $\text{I}$: Definition $22$)