It is a quadrilateral where
both pairs of opposite sides are parallel
i.e. AB ∥ DC and AD ∥ BC
Properties of Parallelogram:

Opposite sides are equal
AB = CD & AD = BC

Opposite angles are equal
∠A = ∠C and ∠B = ∠D

Adjacent angles are supplementary
∠A + ∠B = 180°
∠A + ∠D = 180°
∠C + ∠D = 180°
∠B + ∠C = 180°

Diagonals bisect each other
AO = CO and BO = DO
Proof : Class 9 th − Chapter 8 Theorem 8.6
5.Diagonals divides the Parallelogram into two congruent triangles
∆ ABC ≅ ∆ CDA and
∆ BAD ≅ ∆ DCB
Proof : Class 9 th − Chapter 8 Theorem 8.1