A type of reflection symmetry
in which an object is symmetric
across only one plane
. In other words, there is just one plane you could cut the object with to get two halves
which are mirror image
s of each other.
The letters A, M, T, U, V, W and Y are bilaterally symmetric across a vertical axis, at least with the font that I'm typing with now! The letters B, C, D, and E are bilaterally symmetric in my font, across a horizontal axis. The letters H, I, O and X have two axes of symmetry -- one vertical and one horizontal, so they are beyond bilateral symmetry.
Many animals display bilateral, or near-bilateral, symmetry. Most vertebrates (the mature sole is an exception) and insects have left and right sides which are, more-or-less, mirror images. If you look at internal organs, though, this symmetry often breaks down. For example, in humans, the heart is on the
left, the liver is on the right, and the right lung has three lobes while the left has two.