Symmetry is when you split an object with a line of symmetry into two parts that are exactly similar. So, if I have a circle and draw a line of symmetry in its middle, I would be able to split the circle into two identical parts. Those identical parts are symmetry. Symmetry in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definition, and is usually used to refer to an object that is invariant under some transformations; including translation, reflection, rotation or scaling. In general usage, symmetry most often refers to mirror or reflective symmetry; that is, a line (in 2-D) or plane (in 3-D) can be drawn through an object such that the two halves are mirror images of each other. An isosceles triangle and a human face are examples.