hello
Equilateral parallelograms are also called rhombi (plural of rhombus)

A rhombus is a quadrilateral with all four sides equal. Its opposite sides are parallel.

A more crude way of describing it might be a square with twisted angles, or a skew square.

Like this..
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Sorry I know it is a very crude.. A non-equilateral parallelogram would look like this
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Equilateral (all sides equal) parallelogram. As with a parallelogram, the rhombus has diagonally opposed angles of equal size. Its diagonals bisect each other at right angles, and its area is half the product of the lengths of the two diagonals. The shape is sometimes called a diamond. A rhombus whose internal angles are 90° is called a square.