A rhombus (plural: rhombuses or rhombi) is a parallelogram in which all sides are congruent.
In everyday language, some rhombuses are called diamonds, such as a baseball diamond. Below are examples of said rhombuses/diamonds.

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The term rhomboid can be used to name a parallelogram that has oblique angles and adjacent sides that are unequal. Below is an example of a rhomboid.

Related word rhomboid.

See also parallelogram, square.