Final answer:
A unique property of rhombi, not shared with all parallelograms, is that all sides are of equal length, and its diagonals bisect each other at right angles.
Step-by-step explanation:
One property of rhombi that is not a property of generally defined parallelograms is that all four sides of a rhombus are of equal length. While both a rhombus and a parallelogram have opposite sides that are parallel and equal in length, the additional requirement for a rhombus is that all sides must be congruent to each other. This is not a requirement for a parallelogram. Furthermore, the diagonals of a rhombus bisect each other at right angles (perpendicular), which is another distinctive property of rhombi that does not apply to all parallelograms.