1) It has two pairs of opposite sides that are both parallel and congruent.
(Same with square and rectangle)
2) Its two pairs of opposite angles are congruent.
(Same with square and rectangle)
3) The diagonals bisect each other.
(Same with square and rectangle)
4.) Each diagonal bisects the two opposite angles.
(Same with square and rectangle).
NOW, IF YOU WANT TO PROVE THAT A PARALLELOGRAM IS A RHOMBUS, this is the condition:
IF the diagonals of a parallelogram are perpendicular, THEN it is a rhombus.