What is "parallelogon"

A parallelogon is a polygon such that images of the polygon under translations only tile the plane when fitted together along entire sides.

A parallelogon must have an even number of sides and opposite sides must be equal in length and parallel (hence the name). A less obvious restriction is that a parallelogon can only have four or six sides; a four-sided parallelogon is a parallelogram. In general a parallelogon has 180-degree rotational symmetry around its center.