Symmetry comes from the Greek word symmetrein which roughly translates as "to measure together". One of the most basic forms of symmetry is mirror or line symmetry. In a two dimensional figure, like the photo above, there is a a line that forms an axis of symmetry. On each side of this line there are matching points which are the same distance from the line, or axis of symmetry. So with water the axis of symmetry is the surface of the water. The points of the reflection are the same distance from the surface of the water as the identical points on the log. This is not a perfect example of mirror symmetry though because the water, and the ripple running through it, slightly distort the image.