If, after a rotation, an object looks exactly the same, we say that it has a rotational symmetry.
In a complete turn (of 360o), the number of times an object looks exactly the same is called the order of rotational symmetry.
For example: Consider a rectangle.
Notice that when it is rotated through 360o, it looks exactly the same at two positions.
So, we say that a rectangle has rotational symmetry of order 2.