A four-sided figure is known as a quadrilateral, and one of the most common types of quadrilaterals is the rhombus. A rhombus is a four-sided figure with all sides equal in length and all angles equal in measure. It is also a parallelogram, meaning that opposite sides are parallel and equal in length. Rhombi can be found in nature, such as in the shape of some crystals, and they are also used in art and architecture.