Polygons may have symmetry. Symmetry exists where part of a shape can be folded to overlap exacty the remaining part of the shape.
A polygon may have one or more lines of symmetry. If the polygon was folded along a line of symmetry, then each half of the polygon would match exactly.
A polygon may also have rotational symmetry. If the polygon is rotated about a point and it then looks like itself after being rotated, then it has rotational symmetry. The number of positions where this happens gives the Order of Rotational Symmetry: if it can be rotated three times to look like itself, then it has an Order of Rotational Symmetry of value 3.
A polygon with an order of rotational symmetry equal to 1 does not have rotational symmetry.
How many lines of symmetry does a rectangle have?
A rectangle has two lines of symmetry (it can be folded on two lines such that half of the shape can be matched to the other half).
A line is drawn across a rectangle as shown below. Is it a line of symmetry?
If the rectangle is reflected across the line of symmetry, then it does not match up onto the other side.